Ferroelectric recording medium and ferroelectric storage apparatus

ABSTRACT

A ferroelectric recording medium includes an electrode layer, a ferroelectric recording layer, and a protection layer formed in this order on a substrate, wherein the ferroelectric recording layer includes a ferroelectric layer, and a lattice constant of a material constituting the ferroelectric layer and a lattice constant of a material constituting the electrode layer or the substrate are lattice-matched within a range of ±10%.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Japanese Patent Application No.2020-219654 filed on Dec. 28, 2020, Japanese Patent Application No.2020-219655 filed on Dec. 28, 2020, Japanese Patent Application No.2020-219656 filed on Dec. 28, 2020, Japanese Patent Application No.2020-219657 filed on Dec. 28, 2020, Japanese Patent Application No.2020-219658 filed on Dec. 28, 2020, Japanese Patent Application No.2020-219659 filed on Dec. 28, 2020, Japanese Patent Application No.2020-219660 filed on Dec. 28, 2020, Japanese Patent Application No.2020-219661 filed on Dec. 28, 2020, Japanese Patent Application No.2020-219662 filed on Dec. 28, 2020, Japanese Patent Application No.2020-219663 filed on Dec. 28, 2020, Japanese Patent Application No.2020-219664 filed on Dec. 28, 2020, Japanese Patent Application No.2020-219665 filed on Dec. 28, 2020, Japanese Patent Application No.2021-050640 filed on Mar. 24, 2021, Japanese Patent Application No.2021-050641 filed on Mar. 24, 2021, Japanese Patent Application No.2021-068376 filed on Apr. 14, 2021, Japanese Patent Application No.2021-098592 filed on Jun. 14, 2021, Japanese Patent Application No.2021-101046 filed on Jun. 17, 2021, Japanese Patent Application No.2021-117508 filed on Jul. 16, 2021, and Japanese Patent Application No.2021-160168 filed on Sep. 29, 2021, the entire contents of which areincorporated herein by reference.

BACKGROUND Field

The present invention relates to a ferroelectric recording medium and aferroelectric storage apparatus.

Related Art

As a recording medium for a hard disk drive and various recording media,there is a ferroelectric recording medium that can repeatedly recordinformation by changing the polarization of a ferroelectric. Theferroelectric recording medium is an ultra-high-density recording mediumthat includes a ferroelectric layer and that achieves a high recordingcapacity by using the spontaneous polarization of the ferroelectriccaused by the application of an external electric field. Because thecapacity of the ferroelectric recording medium can be increased, thedevelopment of a ferroelectric storage apparatus including theferroelectric recording medium is being studied.

Patent Document 1 discloses a dielectric recording-and-reproductionapparatus that applies an alternating current electric field to thedielectric material constituting a dielectric recording medium and thatreproduces information recorded in the dielectric recording medium withnon-linear dielectric property of the dielectric material.

Patent Document 2 discloses an information storage apparatus thatincludes a recording medium, including an electrode layer and aferroelectric layer provided on an insulating substrate, attached to aspindle and that records information to the recording medium andreproduces information from the recording medium by holding a headerslider attached to a header assembly above the surface of the recordingmedium by a predetermined distance, and also indicates that asemiconductor sensor is used to read information. Also, Patent Document2 discloses, as the material of the ferroelectric layer, materialshaving Perovskite structures such as lead titanate, barium titanate,strontium titanate, and strontium barium titanate; lithium tantalate;lithium niobate; and the like.

Patent Document 3 discloses an information recording reading header thatincludes a circular guard surrounding the portion around the tip portionto prevent dust from coming into contact with and colliding with theprobe and that moves the probe in a direction substantially orthogonalto the recording surface by using a piezoelectric material as movementmeans.

Patent Document 4 discloses a recording-and-reproduction header thatincludes a protruding portion vertically provided on a support such thatthe tip faces the dielectric recording medium, wherein the protrudingportion has a ridge line at the tip, and the protruding portion isformed using a mold formed by anisotropic etching.

Patent Document 5 discloses a memory apparatus including a dielectriclaminate body formed by laminating a ferroelectric and a paraelectric.

Patent Document 6 discloses a dielectric recording-and-reproductionheader that forms, with a probe and bias electrodes, a polarized domainhaving a polarization direction parallel to the surface of a dielectricrecording medium by applying a voltage, corresponding to data, acrossthe probe and the bias electrodes, so that four types of data arerecorded to a predetermined portion of the dielectric recording medium,and multi-value information is recorded.

RELATED-ART DOCUMENTS Patent Document

-   Patent Document 1: Japanese Patent Laid-Open No. 2004-14016-   Patent Document 2: Japanese Patent Laid-Open No. 2007-272961-   Patent Document 3: Japanese Patent Laid-Open No. 2004-171622-   Patent Document 4: Japanese Patent Laid-Open No. 2005-158117-   Patent Document 5: Japanese Patent Laid-Open No. H9-307073-   Patent Document 6: Japanese Patent Laid-Open No. 2004-178750

Problems to be Solved by the Invention

Progress is being made in storage services that use ultra-high-speedcommunication techniques. In the storage services, various devices suchas computers and communication terminals are connected to the storagevia the Internet, and various kinds of information are shared. Opticalfibers are commonly used for these communications, and the communicationspeeds exceed 10 Gbps (gigabits per second).

In environments where it is difficult to provide optical fibers and inmobiles, wireless and mobile communication over 10 Gbps, which is termedas 5G, is studied, and application to 6G communication over 100 Gbps isalso studied.

In these storage services, recording media such as a hard disk drive(HDD) and a flash memory (such as a solid state drive, SSD) are mainlyused as storage. The transfer speed of the HDD is generally about 1Gbps, and the transfer speed of the SSD is generally about 3 Gbps.Therefore, it is difficult for a single storage to satisfy the strictinput-and-output requirements for ultra-high-speed communication. Inaddition, the capacity required for the storage has been constantlyincreasing.

In addition, as global warming becomes a major social problem, there isconcern that electricity consumption will increase as the use of storageservices expands. Therefore, there is a demand for an efficient storagethat consumes less energy with respect to the unit storage capacity andcan reduce the environmental load.

Ferroelectric storage apparatuses can achieve a high transfer speed anda high storage capacity by laminating a large number of ferroelectricrecording media with a high recording density and rotating them at ahigh speed. For that purpose, it is necessary to manufacture a sharpprobe with good reproducibility, bring the probe close to the surface ofthe recording medium in an order of nanometers, and read and writeinformation.

However, in the dielectric recording-and-reproduction apparatus ofPatent Document 1, when an external electric field is applied to theferroelectric layer, an alternating current electric field is used.Therefore, when information stored in the ferroelectric layer of theferroelectric recording medium is read with the probe, the read speed islimited by the frequency of the alternating current electric field, andtherefore, there is a problem in that information cannot be read at afrequency equal to or more than the frequency of the alternating currentelectric field. Furthermore, dielectric materials with non-lineardielectric properties that can be read at a high speed are limited.Still furthermore, it used to be difficult to manufacture a probe of aheader that can perform reading and writing stably, and it used to bedifficult to control the distance between the probe and theferroelectric layer at a high speed with a high accuracy.

In the reading method of information described in Patent Document 2,there is a problem in that the electric field obtained from theferroelectric layer is weak and accordingly the sensitivity of thesemiconductor sensor is low. In addition, it used to be difficult toform a ferroelectric single-crystal thin film used as a ferroelectriclayer. Furthermore, it used to be difficult to obtain a needle-shapedelectrode with the tip portion sharpened that is used to writeinformation.

In the memory apparatus described in Patent Document 5, it used to bedifficult to detect a weak tunnel current flowing between the probe andthe ferroelectric layer.

In the techniques described in Patent Documents 3, 4, and 6, it has notbeen studied to read, at a high speed, information stored in theferroelectric layer with a high record density.

Therefore, in the conventional techniques, a storage apparatus capableof reading, at a high speed, information stored in the ferroelectriclayer with a high record density cannot be obtained.

SUMMARY

It is an object of one aspect of the present invention to provide aferroelectric recording medium capable of reading, at a high speed,information stored in a ferroelectric recording medium with a highrecord density, capable of reducing the size with respect to the unitstorage capacity, and capable of alleviating the increase in the energyconsumption.

Means for Solving the Problems

An aspect of a ferroelectric recording medium according to the presentinvention is a ferroelectric recording medium that includes an electrodelayer, a ferroelectric recording layer, and a protection layer formed inthis order on a substrate, wherein the ferroelectric recording layerincludes a ferroelectric layer, and a lattice constant of a materialconstituting the ferroelectric layer and a lattice constant of amaterial constituting the electrode layer or the substrate arelattice-matched within a range of ±10%.

Advantageous Effects of the Invention

An aspect of a ferroelectric recording medium according to the presentinvention is capable of reading, at a high speed, information stored ina ferroelectric recording medium with a high record density, capable ofreducing the size with respect to the unit storage capacity, and capableof alleviating the increase in the energy consumption.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross-sectional view illustrating a configuration ofa ferroelectric recording medium according to an embodiment of thepresent invention.

FIG. 2 is a perspective view illustrating the ferroelectric recordingmedium according to the embodiment of the present invention.

FIG. 3 is a cross-sectional view illustrating an example of theferroelectric recording medium according to the embodiment of thepresent invention into which a spindle shaft of an information recordingapparatus is inserted.

FIG. 4 is a drawing illustrating an example of a relationship between acurvature radius of a conductive probe and an electric field strength.

FIGS. 5A to 5D are explanatory diagrams schematically illustrating aprocess as to how polarization inversion of a ferroelectric layerspreads from a central portion directly under the conductive probe to aperipheral portion.

FIGS. 6A and 6B are drawings illustrating data areas and servoinformation areas of the ferroelectric recording medium.

FIG. 7 is a cross-sectional view illustrating an example of aconventional ferroelectric recording medium into which a spindle shaftof an information recording apparatus is inserted.

FIG. 8 is a perspective view illustrating a ferroelectric storageapparatus.

FIG. 9 is a perspective view illustrating a configuration of a headerassembly as seen from the lower side.

FIG. 10 is a cross-sectional view illustrating an example ofconfiguration of a probe slider.

FIG. 11 is a cross-sectional view illustrating another example ofconfiguration of the probe slider.

FIG. 12 is a partially enlarged cross-sectional view of FIG. 11 .

FIG. 13 is a partially enlarged cross-sectional view of FIG. 11 as seenfrom another direction.

FIG. 14 is a partially enlarged view as seen from the lower side of FIG.11 .

FIG. 15 is a cross-sectional view illustrating a configuration of aconductive probe.

FIG. 16 is a drawing illustrating an example of a manufacturing methodof a conductive probe.

FIG. 17 is a drawing illustrating an example of another manufacturingmethod of a conductive probe.

FIG. 18 is a perspective view illustrating another configuration of aconductive probe.

FIG. 19 is a perspective view illustrating another configuration of aconductive probe.

FIG. 20 is a cross-sectional view illustrating another configuration ofa conductive probe.

FIG. 21 is a cross-sectional view illustrating an example ofconfiguration of a probe slider.

FIG. 22 is an explanatory diagram illustrating an example of anothermanufacturing method of a conductive probe.

FIG. 23 is a cross-sectional view illustrating another configuration ofa conductive probe.

FIG. 24 is an explanatory diagram illustrating an example of anothermanufacturing method of a conductive probe.

FIG. 25 is an explanatory diagram illustrating displacement of aconductive probe.

FIG. 26 is a cross-sectional view illustrating a configuration of aferroelectric recording medium driving unit.

FIG. 27 is a drawing illustrating a voltage waveform.

FIG. 28 is a cross-sectional view illustrating an example of anotherconfiguration of a probe slider.

FIG. 29 is an explanatory diagram illustrating an example of aconfiguration of a conventional magnetic header slider.

FIG. 30 is a drawing illustrating a configuration of a data managementsystem according to an embodiment of the present invention.

FIG. 31 is a drawing illustrating an example of a configuration of anexternal storage apparatus.

FIGS. 32A and 32B are explanatory diagrams illustrating an example of aconnection relationship of data saved in a ferroelectric recordingmedium.

DETAILED DESCRIPTION

The embodiment of the present invention is described in detail below. Inorder to facilitate the understanding of the explanation, the samecomponents in the drawings are denoted with the same reference numerals,and duplicate explanation thereabout is omitted. The scales of thecomponents in the drawings may differ from the actual scales. In thepresent specification, a numerical range “A to B” is assumed to beinclusive, i.e., the numerical values A and B recited before and afterthe term “to” are assumed to be included, as the lower limit value andthe upper limit value, respectively, of the numerical range.

<Ferroelectric Recording Medium>

A ferroelectric recording medium according to an embodiment of thepresent invention is explained. FIG. 1 is a partial cross-sectional viewillustrating a configuration of the ferroelectric recording mediumaccording to the embodiment of the present invention. FIG. 2 is aperspective view illustrating the ferroelectric recording mediumaccording to the embodiment of the present invention. FIG. 3 is across-sectional view illustrating an example of a ferroelectricrecording medium according to the embodiment of the present inventioninto which a spindle shaft of an information recording apparatus isinserted. As illustrated in FIG. 1 , a ferroelectric recording medium 10includes a substrate 11, an electrode layer 12, a ferroelectricrecording layer 13, a protection layer 14, and a lubricant layer 15,such that the electrode layer 12, the ferroelectric recording layer 13,the protection layer 14, and the lubricant layer 15 are laminated inthis order on the substrate 11. In proximity to a surface (principalsurface) 101 of the ferroelectric recording medium 10, a conductiveprobe 17 attached to a side of a probe slider 16 facing theferroelectric recording medium 10 is provided, and the conductive probe17 records (writes) information to and reproduces (reads) informationfrom a ferroelectric layer 131 included in the ferroelectric recordinglayer 13.

As illustrated in FIG. 2 , the ferroelectric recording medium 10 isformed in a disk shape including an opening portion 10 a in a centralportion of the principal surface. As illustrated in FIG. 3 , a spindleshaft 18 of a ferroelectric recording medium driving unit for rotatingthe ferroelectric recording medium 10 is inserted into the openingportion 10 a of the ferroelectric recording medium 10, so that theferroelectric recording medium 10 is fixed to the spindle shaft 18.

As illustrated in FIG. 3 , on both of the upper and lower surfaces ofthe substrate 11, the ferroelectric recording medium includesferroelectric recording layers 13 (including the ferroelectric layer 131and the paraelectric layer 132), so that information can be recorded toboth of the upper and lower surfaces of the substrate 11 (double-siderecording). Alternatively, the ferroelectric recording medium 10 mayinclude a ferroelectric recording layer 13 only on one of the uppersurface or the lower surface of the substrate 11, so that information isrecorded to only one side of the substrate 11 (single-side recording).

[Substrate]

The substrate 11 has a function of holding the electrode layer 12, theferroelectric recording layer 13, the protection layer 14, and thelubricant layer 15.

The electrical property of the substrate 11 is not particularly limited,and may be either an insulator or a conductor.

Examples of the insulator include glass, silicon, magnesium oxide (MgO),sapphire, and the like.

Examples of the conductor include: metal materials such as aluminum andits alloys, chromium, platinum, gold, silver, and iron; oxides such asindium oxide (InO₂); and the like. Also, silicon imparted withconductivity by doping can be used.

In a case where the substrate 11 is a conductor, the substrate 11 canachieve the function of the electrode layer 12. Therefore, in a casewhere the electrode layer 12 is not provided on the substrate 11, thematerial constituting the substrate 11 is preferably a conductor.

It is preferable that the substrate 11 does not appreciably havewaviness, has smoothness, and does not appreciably flutter duringhigh-speed rotation.

The thickness of the substrate 11 is not particularly limited so long asthe purpose can be achieved, and is preferably, for example, 100 μm to 1mm.

In view of the lattice matching with the ferroelectric layer 131, thematerial constituting the substrate 11 is preferably such that thelattice constant of the material constituting the ferroelectric layer131 and the lattice constant of the material constituting the substrate11 are lattice-matched within a range of ±10%. Accordingly, thecrystallinity of the ferroelectric layer 131 is increased, and therecording density of the ferroelectric recording medium can beincreased.

The crystal system of the material constituting the substrate 11 ispreferably the same as the crystal system of the material of theferroelectric layer 131. The crystal type of the material constitutingthe substrate 11 is preferably the same as the crystal type of thematerial of the ferroelectric layer 131. The material constituting thesubstrate 11 and the material of the ferroelectric layer 131 may be thesame as each other with respect to any one of the crystal system and thecrystal type, and are, most preferably, the same as each other withrespect to both of the crystal system and the crystal type. Accordingly,the crystallinity of the ferroelectric layer 131 can be increased, andthe recording density of the ferroelectric recording medium can beincreased.

In this case, according to the classification that determines thesymmetry of the crystal, examples of crystal systems include a tricliniccrystal system, a monoclinic crystal system, an orthorhombic crystalsystem, a tetragonal crystal system, a hexagonal crystal system, a cubiccrystal system, and the like. The material constituting the substrate 11and the material of the ferroelectric layer 131 preferably have a samecrystal system.

According to the classification based on close-packed structures ofcrystals, examples of crystal structures include a simple cubic latticestructure, a face-centered cubic lattice structure, a body-centeredcubic lattice structure, a hexagonal close-packed structure, a diamondstructure, a white tin type structure, a graphite structure, an A15 typestructure, a sodium chloride type structure, a cesium chloride typestructure, a zincblende structure, a wurtzite type structure, a nickelarsenide type structure, a lead monoxide type structure, a fluorite typestructure, a pyrite type structure, a cuprite type structure, a rutiletype structure, a cadmium iodide type structure, a bismuth fluoride typestructure, a rhenium oxide type structure, a Ni₄Mo type structure, anAl₄Ba type structure, a calcium boride type structure, a CaCu₅ typestructure, a corundum type structure, a Perovskite type structure, aullmannite type structure, a spinel type structure, a silver phosphatetype structure, a CuAuI type structure, a K4 crystal structure, and thelike. The material constituting the substrate 11 and the material of theferroelectric layer 131 preferably have a same crystal structure.

For example, in a case where hafnium oxide is selected for theferroelectric layer 131, hafnium oxide has the orthorhombic crystalsystem, has the fluorite type structure, and has lattice constants a, b,and c of 5.30 Å, 5.11 Å, and 5.10 Å, respectively. However, the crystalsystem of hafnium oxide is known to change from the monoclinic crystalsystem that is the stable phase to the tetragonal crystal system andfurther to the cubic crystal system. These crystal systems do notexhibit ferroelectricity, and therefore, it is important to changehafnium oxide from the cubic crystal system to the orthorhombic crystalsystem by thermal treatment and the like to make hafnium oxide into aferroelectric.

Therefore, the material constituting the substrate 11 preferably has alattice constant of 4.6 Å to 5.8 Å so as to achieve a lattice-matchwithin a range of ±10%, and the crystal system thereof is preferably anyone of the orthorhombic crystal system, the monoclinic crystal system,the tetragonal crystal system, and the cubic crystal system, and is morepreferably, either the orthorhombic crystal system or the cubic crystalsystem. Also, it is preferable to use a material of which the crystalStructure is the fluorite type structure. Examples of such materialsinclude silicon, Ge, Pd, CeO₂, and the like. Silicon has latticeconstants a, b, and c of 5.4 Å, and has the cubic crystal system. Ge hasa lattice constant of 5.7 Å, and has the cubic crystal system. Pd has alattice constant of 5.0 Å, and has the cubic crystal system. CeO₂ has alattice constant of 5.4 Å, and has the cubic crystal system.

[Electrode Layer]

The electrode layer 12 can be provided on the substrate 11. Theelectrode layer 12 is provided on the lower side of the ferroelectricrecording layer 13 (an opposite side from the conductive probe 17), sothat the electrode layer 12 can function as a counter electrode of theconductive probe 17 that reads information from or writes information tothe ferroelectric recording layer 13.

In view of the lattice match with the ferroelectric layer 131 of theferroelectric recording layer 13, the material constituting theelectrode layer 12 is preferably the same as the ferroelectric layer 131in any one of or both of the crystal system and the crystal type.Accordingly, the crystallinity of the ferroelectric layer 131 can beincreased, and the recording density of the ferroelectric recordingmedium can be increased.

The lattice constant of the material constituting the electrode layer 12is preferably lattice-matched within a range of ±1.0% with respect tothe lattice constant of the material of the ferroelectric layer 131.Accordingly, the crystallinity of the ferroelectric layer 131 can beincreased, and the recording density of the ferroelectric recordingmedium can be increased.

The ferroelectric layer 131 is preferably a single-crystal film.

The material constituting the electrode layer 12 can be appropriatelyselected according to the material of the ferroelectric layer 131, andmay be constituted by, for example, metal materials such as aluminum,chromium, platinum, gold, silver, and iron; oxides such as InO₂; and thelike.

For example, as described above, in a case where hafnium oxide isselected for the ferroelectric layer 131, the material constituting theelectrode layer 12 has a lattice constant of 4.6 Å to 5.8 Å so that thelattice constant achieves a lattice-matched within a range of ±10%, andthe crystal system thereof is preferably any one of the orthorhombiccrystal system, the monoclinic crystal system, the tetragonal crystalsystem, and the cubic crystal system, and is more preferably, either theorthorhombic crystal system or the cubic crystal system. Also, it ispreferable to use a material of which the crystal structure is thefluorite type structure. Examples of such materials include silicon, Ge,Pd, and the like. Ge has a lattice constant of 5.7 Å, and has the cubiccrystal system. Pd has a lattice constant of 5.0 Å, and has the cubiccrystal system.

In this case, because metal materials are more likely to alleviate thelattice strain than oxides, even if the metal materials are differentfrom the ferroelectric layer 131 in the crystal system, the crystaltype, or the lattice constant, the metal materials are likely toalleviate the lattice strain caused by the difference. From thisperspective, the substrate 11 has a higher effect of enhancing thecrystallinity of the ferroelectric layer 131 than the electrode layer12.

Using the material of the electrode layer 12, the electrode layer 12 canbe manufactured by forming a conductive thin film on the substrate 11 byany forming method such as sputtering or thin film deposition.

The thickness of the electrode layer 12 is not particularly limited solong as the electrode layer 12 achieves the purpose, and is preferably,for example, 10 nm to 500 nm.

[Ferroelectric Recording Layer]

The ferroelectric recording layer 13 is provided on the upper surface ofthe electrode layer 12, and has a function of recording information. Theferroelectric recording layer 13 includes the ferroelectric layer 131,and may include other layers.

The ferroelectric layer 131 has a function of recording information. Theferroelectric layer 131 is not particularly limited so long as it is aferroelectric exhibiting ferroelectricity, and is preferably aferroelectric oxide in view of the electrical property.

Examples of ferroelectric oxides include lead titanate (PbTiO₃), leadzirconate (PbZrO₃), barium titanate (BaTiO₃), lithium niobate (LiNbO₃),lithium tantalate (LiTaO₃), hafnium oxide (HfO₂), and the like. Amongthem, in particular, it is preferable to use hafnium oxide.Ferroelectric oxides such as lead titanate, lead zirconate, bariumtitanate, lithium niobate, lithium tantalate, and the like havePerovskite-type crystal structures having the tetragonal crystal systemwith complicated crystal structures and high deposition temperatures. Incontrast, hafnium oxide has a fluorite type structure having theorthorhombic crystal system, and is a binary compound that has a simplerstructure than the Perovskite-type crystal structure, so that hafniumoxide can be deposited at a low temperature.

In a case where the ferroelectric layer 131 includes hafnium oxide, theferroelectric layer 131 preferably includes an additive, or a mixedcrystal (Hf_(x)Zr_(1-x)O₂) including hafnium oxide and zirconium dioxide(ZrO₂).

In a case where the ferroelectric layer 131 includes hafnium oxide andan additive, the additive may be silicon (Si), aluminum (Al), gadolinium(Gd), yttrium (Y), lanthanum (La), strontium (Sr), or the like. One typeof an additive may be used alone, or two or more types of additives maybe used together.

The content of the additive is preferably in a range of 1 atom % to 20atom %, more preferably in a range of 3 atom % to 17 atom %, and stillmore preferably in a range of 5 atom % to 15 atom %. When the content ofthe additive is in the above-described preferred range, the temperatureof the deposition during formation of the ferroelectric layer 131 can bereduced, and the amount of use of the additive can be reduced.

The method of adding the additive to hafnium oxide is not particularlylimited, and any method can be used as appropriate.

In a case where the ferroelectric layer 131 includes a mixed crystal(Hf_(x)Zr_(1-x)O₂) including hafnium oxide and zirconium dioxide, x inHf_(x)Zr_(1-x)O₂ is preferably 0.3 to 0.6.

The ferroelectric layer 131 is preferably constituted by single-crystal,but may include an amorphous structure with short-range order. Theferroelectric layer 131 may be constituted by only the amorphousstructure with short-range order, or may include single-crystal areas.The short-range order is an order of atoms constituting the amorphousstructure over a short distance, and specifically, the short-range ordermeans a property in which the number of nearest atoms, a bond distancebetween atoms, and a bond angle between atoms, and the like are orderly.The long-range order is an order of atoms, constituting the crystal,that are located away from one another, and specifically, the long-rangeorder means that the number of nearest atoms, the bond distance, thebond angle, and the like are in order over a range greatly beyond theinteratomic distance, and a generic term indicating a material havingsuch a structure is single-crystal. When the ferroelectric layer 131includes the amorphous structure with short-range order, thepolarization is inverted by the crystal lattice strain in an areaincluding the amorphous structure, so that information can be recorded.The area including the amorphous structure does not include a grainboundary and a lattice defect, and accordingly, the recording area ofthe ferroelectric layer 131 increases in accordance with an increase inthe area in which the amorphous structure is formed.

The distance of the short-range order of the ferroelectric layer 131 ispreferably equal to or less than 2 nm. The distance of the short-rangeorder is a length of an area in which there is an order between atomsover a short distance, and means a distance in the lengthwise directionand a distance in the widthwise direction between atoms with referenceto the ferroelectric recording medium surface. The distance of theshort-range order is also referred to as a length, a width, and a heightof the short-range order. The bit size of a currently available harddisk drive. (HDD) has a length and a width of about 10 nm on themagnetic recording medium surface, and the area thereof is constitutedby about several magnetic particles. In order for a ferroelectricrecording medium to achieve a higher recording density than a magneticrecording medium, the bit size of the ferroelectric recording mediumneeds to be smaller than the bit size of the magnetic recording medium.In order to provide several recording areas in a single bit area of theferroelectric layer 131, the length of the area having the short-rangeorder (i.e., the length corresponding to a magnetic particle included inthe magnetic recording medium) is preferably equal to or less than 2 nm.

The amorphous structure with short-range order of which the distance isequal to or less than 2 nm can be observed with X-ray diffraction,electron microscope observation, electron diffraction, and the like.Specifically, in such a structure, a clear crystal cannot be observedwith the electron microscope observation, and a blurred intensitydistribution that is referred to as a halo pattern can be obtained withthe electron diffraction. In the X-ray diffraction, a halo pattern isobtained at a crystal plane position having the short-range order. Whenthe ferroelectric layer 131 is heated to a deposition temperature orhigher (i.e., a temperature higher than deposition temperature by +200°C.), the halo pattern changes to a signal having a sharp peak. This isbecause the short-range order that occurred in the ferroelectric layer131 changes into the long-range order due to heating of the substrate.

The lattice constant of the short-range order of the amorphous structureand the lattice constant of the material constituting the substrate 11are preferably lattice-matched within a range of ±10%. Accordingly, theshort-range order of the ferroelectric layer 131 is increased, and therecording density of the ferroelectric recording medium 10 is increased.

The film thickness of the ferroelectric layer 131 can be selected from arange of 1 nm to 1000 nm in total. When the film thickness of theferroelectric layer 131 is in the above-described range, a polarizationinversion can be caused in the ferroelectric layer 131, and the voltagerequired to cause polarization inversion in the ferroelectric layer 131can be reduced.

Also, in order to reduce, as much as possible, the voltage required tocause the polarization inversion of the ferroelectric layer 131 whilecausing the polarization inversion in the ferroelectric layer 131, thefilm thickness of the ferroelectric layer 131 is preferably 1 nm to 30nm, more preferably 3 nm to 25 nm, and still more preferably 5 nm to 20nm in total.

The actual film thickness of the ferroelectric layer 131 is preferablydetermined by comprehensively considering the following points.

With respect to the recording density of the ferroelectric recordingmedium, in a case where the bit length in the track direction is, forexample, 10 nm, the film thickness of the ferroelectric layer 131 ispreferably 10 nm to 50 nm that is one to five times the bit length, andin a case where the bit length is 1 nm, the film thickness is preferably1 nm to 5 nm. One bit of the ferroelectric layer 131 is constituted by asolid, and therefore, based on experiment, it is stabilized when theratio of the length, the width, and the height is within a range of oneto five times.

With respect to read sensitivity with the conductive probe, in a casewhere a tunnel current is used for reading, the film thickness of theferroelectric layer 131 is increased when the read sensitivity isdesired to be increased, and the film thickness of the ferroelectriclayer 131 is decreased when the read sensitivity is desired to bedecreased. Specifically, a ferroelectric is an insulator and has a largeband gap, and therefore, as the film thickness of the ferroelectriclayer 131 increases, the barrier of the energy band increases, andaccordingly, the tunnel current is less likely to flow. Therefore, it isnecessary to increase the sensitivity of the conductive probe detectingthe tunnel current. Conversely, when the film thickness of theferroelectric layer 131 decreases, the tunnel current is more likely toflow, and accordingly, the sensitivity of the conductive probe may bereduced.

In a case where an atomic force is used for reading, the amount ofcharge in the ferroelectric layer 131 increases as the film thickness ofthe ferroelectric layer 131 increases, and therefore, the readsensitivity of the conductive probe detecting the amount of charge maybe reduced. Conversely, when the film thickness of the ferroelectriclayer 131 decreases, the amount of charge in the ferroelectric layer 131also decreases, and therefore, it is necessary to increase thesensitivity of the conductive probe.

With respect to the leak current of the ferroelectric layer 131, as thefilm thickness of the ferroelectric layer 131 increases, the amount ofcharge in the ferroelectric layer 131 also increases, and accordingly,the influence of the leak current can be reduced. Therefore, thefrequency of the refresh (rewrite) can be reduced. Conversely, when thefilm thickness of the ferroelectric layer 131 decreases, the amount ofcharge also decreases, and accordingly, the influence of the leakcurrent also increases, and the frequency of refresh increases.

With respect to the crystallinity of the ferroelectric layer 131, as thefilm thickness of the ferroelectric layer 131 increases, thecrystallinity tends to increase.

With respect to the smoothness of the growth surface of theferroelectric layer 131, the smoothness tends to increase as the filmthickness of the ferroelectric layer 131 decreases.

As a method for forming the ferroelectric layer 131, conventionalmethods can be used. For example, a sputtering method, a chemical vapordeposition (CVD) method, a sol-gel method, a laser ablation method, andthe like may be used. Among them, the sputtering method and the CVDmethod are preferable.

In order to increase the crystallinity of the ferroelectric recordinglayer 13, it is preferable to increase the deposition temperature toabout 500° C. However, when the deposition temperature exceeds 500° C.,the ferroelectric layer 131 is likely to be polycrystallized, and also,the growth surface is also likely to be roughened. Also, the type of thesubstrate 11 that can be used is limited. When the sputtering method andthe CVD method are used as the deposition method, the depositiontemperature can be reduced, the polycrystallization of the ferroelectriclayer 131 can be alleviated, and furthermore, roughening of the growthsurface can be alleviated.

In a case where a sputtering method is used, the method for forming theferroelectric layer 131 is preferably, among sputtering methods, a radiofrequency sputtering method and a reactive sputtering method usingplasma assist in order to increase the electron temperature whilereducing the gas temperature of the deposition area. In a case where theCVD method is used, the method for forming the ferroelectric layer 131is particularly preferably a plasma CVD method, an electron-assistedchemical vapor deposition (EACVD) method, and a metal organic chemicalvapor deposition (MOCVD) method. Accordingly, hafnium oxide is likely tobe changed from the cubic crystal system to the orthorhombic crystalsystem.

As described above, information is recorded in the ferroelectric layer131. The principle for recording and holding information in theferroelectric layer 131 is as follows. Specifically, the ferroelectricconstituting the ferroelectric layer 131 has such a property that thepolarization direction changes in response to an application of anelectric field that exceeds the coercive electric field thereof. Also,the ferroelectric has such a property that, once the polarizationdirection is changed in response to an application of an electric field,the polarization direction is maintained even after the application ofthe electric field is stopped (spontaneous polarization). By making useof this property, information can be recorded and maintained in theferroelectric layer 131. For example, the polarization direction of theentirety of the ferroelectric layer 131 is aligned in a single directionperpendicular to the surface of the ferroelectric recording medium 10.Further, an electric field exceeding the coercive electric field islocally applied to the ferroelectric layer 131 in the directionperpendicular to the surface of the ferroelectric recording medium 10.Accordingly, when the polarization direction of the portion to which theelectric field has been applied is inverted and thereafter theapplication of the electric field is stopped, the polarization directionis maintained as being inverted.

For example, when information to be recorded is digital data of twovalues constituted by “0” and “1”, a bit state “0” is associated with adownward polarization direction, and a bit state “1” is associated withan upward polarization direction. In this case, only when the bit state“1” is recorded, an electric field may be applied to the ferroelectriclayer 131. In this manner, information can be recorded and held in theferroelectric layer 131.

A method for reproducing information recorded in the ferroelectric layer131 as the polarization direction is explained later.

The ferroelectric recording layer 13 may be made into a multi-layerconfiguration by laminating multiple ferroelectric layers 131.

The ferroelectric recording layer 13 preferably includes a singlerecording area of the smallest size in which recording of multi-valueinformation including three or more values (multi-value recording) isperformed in the ferroelectric layer 131 by a ferroelectric storageapparatus explained later (which may be hereinafter simply referred toas a “recording area”). The multi-value recording in the recording areais reproduced by the ferroelectric storage apparatus. Informationrecorded in the recording area is made into multi-value, so that therecording density of the ferroelectric layer 131 can be enhanced.

The multi-value recording is a method for recording information of threeor more values in the recording area of the smallest size. For example,in a magnetic recording layer of a magnetic recording medium such as anHDD, the recording area of the smallest size is constituted by magneticpoles of two values, i.e., a north pole and a south pole. In contrast,the recording area of the smallest size of the ferroelectric recordinglayer 13 is preferably constituted by three or more values.

In the ferroelectric layer 131, information is recorded by polarization.For example, it is assumed that the front surface side of theferroelectric layer 131 is spontaneously polarized as positive, and theback surface side of the ferroelectric layer 131 is spontaneouslypolarized as negative. In this case, when a positive electric field of acertain strength or higher is generated at the tip of the conductiveprobe 17 provided to face this portion, the polarization of this portioncan be inverted such that the front surface side is made into negativeand the back surface side is made into positive.

In general, a conductive electrode with a sharpened needle shape is usedas the conductive probe 17. FIG. 4 illustrates an example of arelationship between a curvature radius r of the tip portion of theneedle-shaped electrode and an electric field strength E that occurs inthe space of the tip. As illustrated in FIG. 4 , the curvature radius rof the tip portion of the needle-shaped electrode and the electric fieldstrength E that occurs in the space of the tip are inverse proportionalto each other, and the electric field strength E increases in accordancewith a decrease in the curvature radius r. The conductive probe 17 has acone shape in normal circumstances and its tip portion is spherical froma microscopic perspective, with the radius r at the end of the tipportion being the smallest and the radius r increasing gradually awayfrom the end of the tip portion, and accordingly, the electric fieldstrength E is the highest at the position of the conductive probe 17directly under the tip portion, and the electric field strength Edecreases away from the position directly under the tip portion.Therefore, when the voltage applied to the conductive probe 17 isgradually increased, the polarization inversion of the ferroelectriclayer 131 expands from the central portion directly under the conductiveprobe 17 to the peripheral portion.

FIG. 5A to FIG. 5D are explanatory diagrams schematically illustrating aprocess for inverting the polarization of the ferroelectric recordinglayer. As illustrated in FIG. 5A to FIG. 5D, the conductive probe 17 iscaused to face, in a non-contact state, the surface of the ferroelectriclayer 131 spontaneously polarized to the positive (see FIG. 5A). When apositive voltage is applied to the conductive probe 17, and the electricfield strength that occurs in the space at the tip portion due to theapplied voltage exceeds the polarization inversion potential of theferroelectric layer 131, first, the polarization of the positiondirectly under the conductive probe 17 is inverted to the negative (seeFIG. 5B). Then, when the applied voltage is further increased, thepolarization inversion to the negative expands from the positiondirectly under the conductive probe 17 to the peripheral portion (seeFIG. 5C and FIG. 5D).

In this case, five consecutive charging positions as illustrated in FIG.5A to FIG. 5D are assumed to be a recording area of the smallest size.In this case, the number of positive charges in a single recording areais five in FIG. 5A, four in FIG. 5B, two in FIG. 5C, and zero in FIG.5D. According to the number of positive charges, multi-value informationof four values is recorded. Write of multi-value information to therecording area of the smallest size is performed by a simplest singlewrite operation by the ferroelectric storage apparatus.

Information multi-value recorded in the ferroelectric layer 131(multi-value information) is read and reproduced by the ferroelectricstorage apparatus. The reproduction (read) of the multi-valueinformation is performed by a simplest single read operation by theferroelectric storage apparatus.

Examples of methods for reproducing information recorded in theferroelectric layer 131 include a method using a difference in thedielectric constant of the ferroelectric layer 131 caused by thepolarization direction of the ferroelectric layer 131, a method fordetecting a weak tunnel current flowing between the conductive probe 17and the electrode layer 12, and a method for detecting an atomic forcebetween the conductive probe 17 and the ferroelectric layer 131, and thelike. In any of the methods, multi-value recorded information can bereproduced from the amount of charge in the recording area of thesmallest size of the ferroelectric layer 131.

Specifically, in a case where the using a difference in the dielectricconstant of the ferroelectric layer 131 caused by the polarizationdirection of the ferroelectric layer 131 is employed, the difference inthe dielectric constants also increases in accordance with an increasein the difference between the amount of positive charge and the amountof negative charge in a single recording area.

In a case where the method for detecting a weak tunnel current flowingbetween the conductive probe 17 and the electrode layer 12 is employed,the tunnel barrier of the ferroelectric layer 131 changes according tothe polarization direction and the amount of polarization. The tunnelcurrent injected from the electrode layer 12 also changes, and theinformation multi-value recorded in the ferroelectric recording medium10 can be reproduced by detecting the amount of change.

In a case where the method for detecting an atomic force between theconductive probe 17 and the ferroelectric layer 131 is employed, theelectric force (Maxwell stress) between the ferroelectric layer 131 andthe conductive probe 17 changes according to the polarization directionand the amount of polarization of the ferroelectric layer 131. Theinformation multi-value recorded in the ferroelectric recording medium10 can be reproduced by detecting the amount of change as well as theatomic force.

In the ferroelectric storage apparatus, position information (alsoreferred to as “servo information”) for detecting a relative positionbetween the conductive probe 17 and the ferroelectric recording medium10 in the track direction of the ferroelectric recording medium 10 ispreferably recorded in the ferroelectric layer 131. In this case, in theferroelectric layer 131, servo information areas recorded with the servoinformation and areas for recording and reproducing data are preferablyarranged alternately with regular intervals in the circumferentialdirection of the track. Accordingly, the conductive probe 17 can detectthe position of the conductive probe 17 according to the servoinformation during reproduction of recorded data.

FIGS. 6A and 6B are drawings illustrating data areas and servoinformation areas of the ferroelectric layer 131. FIG. 6A is a plan viewillustrating the ferroelectric layer 131. FIG. 6B is an enlarged view ofa rectangular area A of FIG. 6A. As illustrated in FIG. 6A and FIG. 6B,on one of the surfaces of the electrode layer 12 in the disk shape, theferroelectric layer 131 may include data areas 131A and servoinformation areas 131B. In FIG. 6A, areas indicated by lines extendingradially from the center are the servo information areas 131B, and areasbetween radial lines are data areas 131A. As illustrated in FIG. 6B, thedata areas 131A have annular-like regular shapes.

As illustrated in FIG. 6B, the servo information area 131B includes aburst information area 131B-1, an address information area 131B-2,preamble information area 131B-3, and reference signal information131B-4. In FIG. 6B, for example, the conductive probe 17 travels fromleft to right, but the order for arranging the burst information area131B-1, the address information area 131B-2, the preamble informationarea 131B-3, and the reference signal information 131B-4 may be changedas necessary.

The burst information area 131B-1 is recorded with burst information andthe like for positioning the conductive probe 17 at the center of therecording track.

The address information area 131B-2 is recorded with address informationincluding track information (information in the radius direction) andsector information (information in the circumferential direction)indicating an address of the data area 131A.

The preamble information area 131B-3 and the reference signalinformation 131B-4 are recorded with preamble information used foridentifying a position for transitioning from the data area 131A to theservo information area 131B in the recording track.

In the ferroelectric layer 131 as illustrated in FIG. 6A, the conductiveprobe 17 moving in the circumferential direction on the surface preparesto read address information by reading the preamble information of thepreamble information area 131B-3. Then, the conductive probe 17 readsthe address information of the data area 131A from the addressinformation area 131B-2, and reads the burst information from the burstinformation area 131B-1, thereby fine-adjusting the track position (theradius position). Thereafter, the conductive probe 17 can recordinformation to and read information from the data area 131A.

The servo information area 131B preferably includes the reference signalinformation 131B-4 indicating the reference of the signal level of themulti-value recorded information. For example, in the four-valuerecording of FIG. 5 , the reference signal information 131B-4 mayinclude recording of four types of amounts of charges corresponding toFIG. 5A, FIG. 5B, FIG. 5C, and FIG. 5D. Alternatively, the referencesignal information 131B-4 may include recording of two types of amountsof charges corresponding to FIG. 5A and FIG. 5D, and FIG. 5B and FIG. 5Cmay be obtained as ⅓ and ⅔, respectively, of the difference of signallevels between FIG. 5A and FIG. 5D.

The conductive probe 17 reads the reference signal information 131B-4from the servo information area 131B, so that the signal levels ofmulti-value recording are recognized, and the multi-value informationrecorded in the data area 131A is reproduced by using the recognizedsignal levels.

As illustrated in FIG. 1 , the ferroelectric recording layer 13preferably includes the paraelectric layer 132.

The paraelectric layer 132 is preferably provided on a side of theferroelectric recording layer 13 facing the electrode layer 12, andprovided between the ferroelectric layer 131 and the electrode layer 12.

In a case where the ferroelectric recording layer 13 is constituted by asingle layer including only the ferroelectric layer 131, theferroelectric layer 131 is provided to be in contact with the electrodelayer 12, but in this case, charge of the polarized ferroelectric layer131 leaks to the electrode layer 12, and information recorded in theferroelectric layer 131 may be lost.

In particular, in an initial growth area of the ferroelectric layer 131,i.e., an area in proximity to the interface with the electrode layer 12,the crystallinity may be reduced during deposition, and for example, apolycrystal or amorphous structure is likely to be formed. For thisreason, charge is likely to be leaked from this polycrystal or amorphousstructure portion to the electrode layer 12. Specifically, this isbecause the amorphous structure portion formed at the interface of theferroelectric layer 131 is completely amorphous without any long-rangeorder or short-range order, and accordingly, leakage of charge is likelyto occur similarly to the grain boundary portion of a polycrystal.

Because the insulating paraelectric layer 132 is provided between theferroelectric layer 131 and the electrode layer 12, leakage of chargefrom the ferroelectric layer 131 can be reduced.

In addition, because the insulating paraelectric layer 132 is providedbetween the ferroelectric layer 131 and the electrode layer 12, there isan effect of increasing the tunnel current from the ferroelectric layer131.

A ferroelectric is an insulator with a large band gap, and therefore, atunnel current is less likely to flow. However, when a ferroelectric ismade into a thin film, the tunnel barrier can be reduced, and a bondingstructure with the electrode layer that is a conductor can be formed, sothat a weak tunnel current can be passed from the electron state at thebonding portion. Furthermore, when a paraelectric layer is added to thebonding portion, and accordingly, the bonding structure includes theferroelectric, the paraelectric, and the conductor which are bonded inthis order, band bending in a direction to reduce the tunnel barrieroccurs due to charge at the interface portion between the ferroelectricand the paraelectric, and the tunnel current is more likely to flow.

In the paraelectric layer 132, a conventional material can be used asthe paraelectric. For example, oxide, nitride, carbide, boride, andsilicide are preferably used as the paraelectric. One of theabove-described materials may be used alone, or two or more may be usedtogether.

Examples of oxides include alumina, zirconia, yttria-stabilizedzirconia, silicon oxide, titanium oxide, cerium oxide, titanium oxide,lead oxide, yttrium oxide, barium oxide, chromium oxide, iron oxide,lanthanum oxide, and the like.

Examples of nitrides include titanium nitride, silicon nitride, chromiumnitride, aluminum nitride, and the like.

Examples of the carbides include titanium carbide, tungsten carbide;boron carbide, silicon carbide, chromium carbide, and the like.

Examples of borides include titanium boride, iron tetraboride, neodymiumboride, and the like.

Examples of silicides include molybdenum silicate and the like.

Similar to the substrate 11 and the electrode layer 12, the paraelectricconstituting the paraelectric layer 132 is preferably a material havingany one of or both of the same crystal system and the same crystalstructure as the crystal system and the crystal structure of theferroelectric layer 131, in view of lattice match with the ferroelectriclayer 131.

Similar to the substrate 11 and the electrode layer 12, the latticeconstant of the paraelectric constituting the paraelectric layer 132 ispreferably lattice-matched within a range of ±10% with respect to thelattice constant of the ferroelectric constituting the ferroelectriclayer 131. Accordingly, the ferroelectric layer 131 is likely to grow onthe paraelectric layer 132, and the crystallinity of the ferroelectriclayer 131 can be increased.

For example, as described above, in a case where hafnium oxide isselected for the ferroelectric layer 131, the material constituting theparaelectric layer 132 has a lattice constant of 4.6 Å to 5.8 Å so thatthe lattice constant achieves lattice matching within a range of ±10%,and the crystal system thereof is preferably any one of the orthorhombiccrystal system, the monoclinic crystal system, the tetragonal crystalsystem, and the cubic crystal system, and is more preferably, either theorthorhombic crystal system or the cubic crystal system. Also, it ispreferable to use a material of which the crystal structure is thefluorite type structure.

Examples of such materials include cerium oxide (cubic crystal system,fluorite type structure, lattice constant 5.4 Å), silicon (cubic crystalsystem, diamond type structure, lattice constant 5.4 Å), 10 (Y₂O₃)-90(ZrO₂) (cubic crystal system, fluorite type structure, lattice constant5.1 Å), aluminum oxide (trigonal system, corundum type structure,lattice constant 4.8 Å), titanium oxide (tetragonal crystal system,rutile type structure, lattice constant 4.6 Å), and the like.

The effect of the paraelectric layer 132 for increasing thecrystallinity of the ferroelectric layer 131 by lattice match with theferroelectric layer 131 is almost as high as the effect achieved by thesubstrate 11 and is higher than the effect achieved by the electrodelayer 12. Also, as described above, at the interface portion between theferroelectric layer 131 and the paraelectric layer 132, band bending ina direction for reducing the tunnel barrier occurs, but this bandbending is strongly affected by the lattice strain at the interfaceportion. Specifically, the lattice strain increases the energy at theinterface portion, and accordingly, this increased energy may hide theenergy state due to the above-described band bending. Therefore, inorder to detect a weak current flowing between the conductive probe 26and the ferroelectric layer 131, it, is important to increase thelattice match between the paraelectric layer 132 and the ferroelectriclayer 131, and reduce the energy increase due to the lattice strain.

The film thickness of the paraelectric layer 132 is preferably 1 nm to100 nm, and more preferably, 5 nm to 50 nm in view of reduction ofleakage of charge from the ferroelectric layer 131.

When the film thickness of the paraelectric layer 132 is within theabove-described preferred range, polarization inversion can be caused inthe ferroelectric layer 131, leakage of charge from the ferroelectriclayer 131 can be reduced, and leakage of charge from the ferroelectriclayer 131 to the outside can be reduced. Furthermore, when the filmthickness of the paraelectric layer 132 is within the above-describedpreferred range, the tunnel current is more likely to flow, andaccordingly, the tunnel current can be increased.

In addition, the increase in the distance between the ferroelectriclayer 131 and the electrode layer 12 can be alleviated, so that thevoltage required to invert the polarization of the ferroelectric layer131 can be reduced. Therefore, while the increase in the film thicknessof the ferroelectric recording layer 13 is alleviated, an increase inthe voltage required to invert the polarization of the ferroelectriclayer 131 can be alleviated.

In order to increase the tunnel current from the ferroelectric layer131, the film thickness of the paraelectric layer 132 is preferably 1 nmto 30 nm, and more preferably, about the same as the film thickness ofthe ferroelectric layer 131. In this case, the film thickness of theferroelectric layer 131 is also preferably 1 nm to 30 nm in total. Thefilm thickness of the paraelectric layer 132 is preferably the same asthe total film thickness of the ferroelectric layer 131. In a case wherethere is difference between the film thickness of the paraelectric layer132 and the total film thickness of the ferroelectric layer 131, theparaelectric layer 132 is preferably thinner than the total filmthickness of the ferroelectric layer 131, and the difference between thefilm thickness of the paraelectric layer 132 and the total filmthickness of the ferroelectric layer 131 is preferably equal to or lessthan 10 nm.

[Protection Layer]

The protection layer 14 is provided on the upper surface of theferroelectric recording layer 13. The protection layer 14 has a functionof protecting the ferroelectric recording layer 13 from the outside, andeven if, e.g., the ferroelectric recording medium 10 comes into contactwith the conductive probe 17, damage to the ferroelectric recordinglayer 13 can be alleviated.

The protection layer 14 is preferably an insulating material with a lowdielectric constant, in order not to impair the function of theferroelectric recording layer 13 for recording and reproducinginformation.

The protection layer 14 may be constituted by: oxides such as silica,alumina, zirconia, titania, magnesium oxide, aluminum oxide, and thelike; nitrides such as silicon nitride, aluminum nitride, titaniumnitride, boron nitride, and the like; carbides such as silicon carbide,boron carbide, and the like; a diamond-like carbon film; a polymerinsulating material; or the like. In order to protect the ferroelectricrecording layer and alleviate the reduction of its durability, theprotection layer 14 is preferably made of a material with a high degreeof hardness.

The thickness of the protection layer 14 is preferably equal to or morethan 0.5 nm. In view of the voltage required for polarization inversionof the ferroelectric of the ferroelectric layer 131, the thickness ofthe protection layer 14 is preferably equal to or less than 50 nm, morepreferably equal to or less than 30 nm, and still more preferably equalto or less than 20 nm.

As illustrated in FIG. 3 , the ferroelectric recording medium 10preferably does not include the ferroelectric recording layer (theferroelectric layer 131 and the paraelectric layer 132) and theprotection layer 14 around the opening portion 10 a. Specifically, theferroelectric recording layer 13 and the protection layer 14 arepreferably provided in an area other than the opening portion 10 a andan area therearound at the center of the ferroelectric recording medium10. The area around the opening portion 10 a means a range of a distanceof about 10 mm from the inner circumference of the opening portion 10 a,although it depends on the size of the substrate 11, the size of theopening portion 10 a, and the like. The ferroelectric recording layer 13and the protection layer 14 are provided in the area other than theopening portion 10 a and the area therearound, so that the ferroelectricrecording layer 13 and the protection layer 14 do not come into contactwith the spindle shaft 18, and leakage of charge of the ferroelectriclayer 131 through the protection layer 14 to the spindle shaft 18 can bealleviated.

The ferroelectric recording medium 10 has a structure obtained bylaminating the ferroelectric recording layer 13, the protection layer14, and the lubricant layer 15 over the substrate 11 and both of thesurfaces of the electrode layer 12. The spindle shaft 18, to which theferroelectric recording medium 10 is attached, is formed with a step ina cylindrical shape made by reducing the diameter at the upper portion,and the ferroelectric recording medium 10 is fixed to the spindle shaft18 by placing the ferroelectric recording medium 10 on the step, placingan attachment metal fitting 19 thereon, and screwing the attachmentmetal fitting 19 to the spindle shaft 18. At the portion where theferroelectric recording medium 10 is fixed to the spindle shaft 18, theferroelectric recording layer 13, the protection layer 14, and thelubricant layer 15 are not provided, and the substrate 11 and thespindle shaft 18 are directly in contact with each other. The spindleshaft 18 is connected via the substrate 11 to the ferroelectric layer131, and constitutes a part of a circuit for reading information fromand writing information to the ferroelectric layer 131.

For example, as illustrated in FIG. 7 , in a case where a ferroelectricrecording medium of which the entire surface of the substrate 11 isprovided with the electrode layer 12, the ferroelectric layer 131, andthe protection layer 14 is attached to the spindle shaft 18, theferroelectric layer 131 has an insulating property, and therefore, evenif the ferroelectric layer 131 is in contact with the spindle shaft 18,it is impossible to read information from and write information to theferroelectric recording medium 10. However, because the ferroelectricrecording layer 13 and the protection layer 14 are thin, a conductionwith the substrate 11 can be achieved by making a conduction between thesubstrate 11 and the spindle shaft 18 by causing a protruding portion181 a of an attachment surface 181 of the spindle shaft 18 to penetratethe ferroelectric layer 131 and the protection layer 14 and bite intothe electrode layer 12.

When the ferroelectric recording medium 10 is set in the ferroelectricstorage apparatus and used over a long period of time, the protectionlayer 14 deteriorates due to moisture and contamination in theferroelectric storage apparatus, and the insulating property of theprotection layer 14 decreases, so that charge of the ferroelectric layer131 of the ferroelectric recording layer 13 may leak. Even though thedeterioration of the ferroelectric recording layer 13 is less severebecause the ferroelectric recording layer 13 is covered with theprotection layer 14, the ferroelectric recording layer 13 may alsodeteriorate similar to the protection layer 14, and charge may leak fromthe ferroelectric layer 131. In the present embodiment, as illustratedin FIG. 3 , the ferroelectric recording layer 13 and the protectionlayer 14 are formed in an area other than the opening portion 10 a ofthe ferroelectric recording medium 10 and the area therearound, so thatcharge of the ferroelectric recording layer 13 can be prevented fromleaking from the protection layer 14 to the spindle shaft 18.

As illustrated in FIG. 1 , the lubricant layer 15 may be provided on thesurface of the protection layer 14 in order to alleviate abrasion causedby contact with the conductive probe 17.

The lubricant used for the lubricant layer 15 is preferably saturatedfatty acid such as stearic acid, dye such as phthalocyanine, fluorineresin such as perfluoropolyether (PFPE), and is particularly preferablyfluorine resin such as PFPE, because the lubricity is high.

The lubricant layer 15 is preferably an insulating material with a lowdielectric constant, in order not to impair the function of theferroelectric recording layer 13 for recording and reproducinginformation.

In this manner, in the ferroelectric recording medium 10 according tothe present embodiment, the lattice constant of the material of theferroelectric layer 131 and the lattice constant of the materialconstituting the electrode layer 12 are lattice-matched within a rangeof ±10%. Accordingly, the crystallinity of the ferroelectricconstituting the ferroelectric layer 131 can be increased. Furthermore,the ferroelectric included in the ferroelectric layer 131 is made intosingle-crystal, and the grain boundary is eliminated, so that theinfluence of the grain boundary can be reduced. The polarizationinversion of the ferroelectric included in the ferroelectric layer 131is caused by the crystal lattice strain, and accordingly, the area ofthe ferroelectric layer 131 where the polarization of the ferroelectricis inverted can be increased by making the ferroelectric included in theferroelectric layer 131 into single-crystal. Therefore, theferroelectric recording medium 10 can achieve a high recording density.

In a case where the substrate 11 is a conductor, the substrate 11 isconductive, and therefore, the ferroelectric recording medium 10 canalso serve as an electrode. In this case, the substrate 11 can achievethe function as the electrode layer 12, and the electrode layer 12 isunnecessary. Therefore, even though the ferroelectric recording medium10 does not have the electrode layer 12 on the substrate 11, the latticeconstant of the material of the ferroelectric layer 131 and the latticeconstant of the material constituting the substrate 11 can belattice-matched within a range of ±10%. Therefore, as described above,the ferroelectric recording medium 10 can increase the area where thepolarization of the ferroelectric is inverted in the ferroelectric layer131, and therefore, the ferroelectric recording medium 10 can achieve ahigh recording density.

In the ferroelectric recording medium 10, the ferroelectric layer 131may be a single-crystal film. Accordingly, the grain boundary of theferroelectric layer 131 can be eliminated, so that the recordingcapacity of the ferroelectric recording medium 10 can be furtherincreased.

The ferroelectric recording medium 10 includes the ferroelectric layer131, the ferroelectric layer 131 includes the amorphous structure withshort-range order, and the distance of the short-range order is set to 2nm or less, so that the lattice constant of the amorphous structure andthe lattice constant of the material constituting the substrate 11 canbe lattice-matched within a range of ±10%. Because the ferroelectriclayer 131 has the amorphous structure with the short-range order,polarization inversion due to crystal lattice strain can be caused inthe area having the short-range order, and the grain boundary can bereduced. Therefore, in the ferroelectric recording medium 10, a largearea of the ferroelectric layer 131 can be made into a recording area.Furthermore, because the distance of the short-range order of theferroelectric layer 131 is set to 2 nm or less, the storage area can beincreased, and accordingly, the recording density can be increased.Therefore, the ferroelectric recording medium 10 can achieve a highrecording density.

In a case where the ferroelectric layer 131 has the amorphous structurewith short-range order, as compared with a case where the ferroelectriclayer 131 is formed by a single-crystal film, the deposition temperatureof the ferroelectric layer 131 can be further reduced. Accordingly, thegrowth surface of the ferroelectric layer 131 becomes smooth, so thatthe surface of the ferroelectric recording medium 10 can be made smooth.Therefore, in the ferroelectric recording medium 10, the spacing losswith respect to the conductive probe 17 can be reduced, and accordingly,the recording density can be further increased.

In the ferroelectric recording medium 10, the ferroelectric layer 131has the amorphous structure with short-range order, and accordingly, thedeposition temperature of the ferroelectric layer 131 can be furtherreduced, so that a larger number of types of substrates are availablefor the substrate 11.

In the ferroelectric recording medium 10, the ferroelectric layer 131has the amorphous structure with short-range order, and accordingly, theferroelectric layer 131 can be readily made into a thin film, so that asmooth surface can be readily formed on the ferroelectric layer 131.Therefore, the smoothness of the ferroelectric recording medium 10 canbe increased.

Further, in the ferroelectric recording medium 10, the ferroelectriclayer 131 includes the amorphous structure with short-range order, andaccordingly, the grain boundary of the ferroelectric layer 131 can bereduced, so that the recording capacity with respect to the singleferroelectric recording medium 10 can be increased.

In the ferroelectric recording medium 10, the substrate 11 can havesilicon, and the ferroelectric layer 131 can have hafnium oxide. Thelattice match property between the substrate 11 and the ferroelectriclayer 131 can be increased, and the crystallinity of the ferroelectriclayer 131 can be increased. Therefore, the ferroelectric recordingmedium 10 can reliably achieve a higher recording density.

In the ferroelectric recording medium 10, the ferroelectric layer 131may be constituted by a mixture including hafnium-oxide and at least oneadditive selected from the group comprising silicon, aluminum,gadolinium, yttrium, lanthanum, and strontium, or may be constituted bymixed crystal (Hf_(x)Zr_(1-x)O₂) (x is 0.3 to 0.6) including hafniumoxide and zirconium dioxide. Accordingly, when the ferroelectric layer131 is made into the above-described mixture or mixed crystal includinghafnium oxide, the deposition temperature can be reduced duringformation of the ferroelectric layer 131 on the substrate 11, so thatthe ferroelectric layer 131 can be formed on the substrate 11 at a lowtemperature. In general, the higher the temperature is during formationof the ferroelectric layer 131, the more readily the ferroelectric layer131 can be polycrystallized. In the present embodiment, theferroelectric enables increase of the single-crystal area, so that thesingle-crystal area of the ferroelectric layer 131 can be increased moresignificantly. Therefore, the ferroelectric recording medium 10 canachieve a higher recording capacity.

In the ferroelectric recording medium 10, the single-crystal area of theferroelectric layer 131 is increased, and accordingly, the ferroelectricrecording medium 10 can be readily made into a thin film, and a smoothsurface can be formed over a wider range in the ferroelectric layer 131,so that the smoothness of the ferroelectric layer 131 can be furtherincreased.

In the ferroelectric recording medium 10, the content of the additivecan be made into a range of 1 atom % to 20 atom %. Accordingly, in theferroelectric recording medium 10, the ferroelectricity of theferroelectric layer 131 can be increased.

In the ferroelectric recording medium 10, the film thickness of theferroelectric layer 131 can be made into 5 nm to 1000 nm. Accordingly,in the ferroelectric recording medium 10, the polarization of theferroelectric included in the ferroelectric layer 131 can be inverted,and the voltage required for inverting the polarization of theferroelectric can be reduced, so that the burden imposed on theferroelectric layer 131 can be alleviated.

The ferroelectric recording medium 10 may have the paraelectric layer132 between the ferroelectric recording layer 13 and the electrode layer12, and the paraelectric layer 132 may include at least one paraelectricselected from the group comprising oxide, nitride, carbide, boride, andsilicide. During formation of the ferroelectric layer 131, the initialgrowth area, i.e., an area in proximity to the interface with theelectrode layer 12, is polycrystal or amorphous, the crystallinityincreases toward the growth direction of the film thickness, andultimately, the surface is made into single-crystal. Therefore, chargeof the ferroelectric layer 131 is likely to leak from the polycrystal oramorphous area in proximity to the interface to the side of thesubstrate 11. The paraelectric layer 132 can alleviate leakage of thecharge of the ferroelectric layer 131 to the substrate 11. Therefore, inthe ferroelectric recording medium 10, the leakage of the charge fromthe ferroelectric layer 131 can be alleviated, so that loss ofinformation recorded in the ferroelectric layer 131 can be alleviated.

In addition, in the ferroelectric recording medium 10, the latticeconstant of the material constituting the paraelectric layer 132 and thelattice constant of the material of the ferroelectric layer 131 can belattice-matched within a range of ±10%. Accordingly, the crystallinityof the ferroelectric layer 131 can be increased more greatly, and theleakage of the charge from the ferroelectric layer 131 can be alleviatedmore significantly.

Furthermore, in the ferroelectric recording medium 10, the filmthickness of the paraelectric layer 132 can be made into 1 nm to 100 nm.Accordingly, in the ferroelectric recording medium 10, while thepolarization of the ferroelectric layer 131 is inverted, the effect ofalleviating the leakage of the charge from the ferroelectric layer 131can be further enhanced.

In the ferroelectric recording medium 10, the film thickness of theparaelectric layer 132 can be made into 1 nm to 30 nm, the filmthickness of the ferroelectric layer 131 can be made into 1 nm to 30 nm,the film thickness of the paraelectric layer 132 and the film thicknessof the ferroelectric layer 131 can be made substantially the same aseach other or the film thickness of the paraelectric layer 132 can bemade smaller than the total film thickness of the ferroelectric layer131, and the difference in the film thickness between the paraelectriclayer 132 and the ferroelectric layer 131 can be made 10 nm or less.Accordingly, when the ferroelectric recording medium 10 is applied tothe ferroelectric storage apparatus, a weak tunnel current flowingbetween the conductive probe 17 and the ferroelectric layer 131 can beincreased.

The ferroelectric recording medium 10 can have the opening portion 10 aat the central portion of the substrate 11, and the protection layer 14can be provided in the area other than the opening portion 10 a and thearea therearound. The protection layer 14 does not come into contactwith the spindle shaft 18, and therefore, even if the protection layer14 deteriorates over an elapse of time, leakage of the charge of theferroelectric layer 131 through the protection layer 14 to the spindleshaft 18 can be alleviated in the ferroelectric recording medium 10.Therefore, in the ferroelectric recording medium 10, the leakage of thecharge from the ferroelectric layer 131 can be alleviated, so that lossof information recorded in the ferroelectric layer 131 can bealleviated, and the adverse effect caused by the deterioration of theprotection layer 14 over an elapse of time can be alleviated.

In the ferroelectric recording medium 10, the ferroelectric recordinglayer 13 can be provided in the area other than the opening portion 10 aand the area therearound. Accordingly, the ferroelectric recording layer13 does not come into contact with the spindle shaft 18, so that even ifthe ferroelectric recording layer 13 and the protection layer 14deteriorate over an elapse of time, leakage of the charge of theferroelectric layer 131 through the ferroelectric recording layer 13 andthe protection layer 14 to the spindle shaft 18 can be alleviated in theferroelectric recording medium 10. Therefore, in the ferroelectricrecording medium 10, the leakage of the charge from the ferroelectriclayer 131 can be alleviated more reliably, so that loss of informationrecorded in the ferroelectric layer 131 can be alleviated in a morestable manner, and the adverse effect caused by deterioration of theferroelectric recording layer 13 and the protection layer 14 over anelapse of time can be alleviated in a more effective manner.

<Ferroelectric Storage Apparatus>

A ferroelectric storage apparatus having the ferroelectric recordingmedium 10 according to the present embodiment is explained. Theferroelectric storage apparatus includes, as a ferroelectric recordingmedium, the above-described ferroelectric recording medium 10. FIG. 8 isa perspective view illustrating the ferroelectric storage apparatus. Asillustrated in FIG. 8 , the ferroelectric storage apparatus 100 includesthe ferroelectric recording medium 10, a header assembly 20, aferroelectric recording medium driving unit 30, a probe driving unit 40,a control unit, not illustrated, and a recording-and-reproduction signalprocessing unit 50, which are provided in a housing 60. Theferroelectric storage apparatus 100 includes multiple ferroelectricrecording media 10. Because the ferroelectric recording medium 10 is theabove-described ferroelectric recording medium according to the presentembodiment, detailed explanation thereabout is omitted.

[Header Assembly]

As illustrated in FIG. 8 , the header assembly 20 is fixed to a fixedshaft in the ferroelectric storage apparatus 100, and includes anactuator arm 21, a suspension arm 22, and a probe slider 23.

FIG. 9 is a perspective view illustrating a configuration of the headerassembly 20 as seen from the lower side. As illustrated in FIG. 9 , leadwires 24 for writing and reading signals are arranged on the suspensionarm 22. One end of each of the lead wires is electrically connected tothe conductive probe 26 incorporated into the probe slider 23, and theother end of each of the lead wires is electrically connected to theelectrode pad 25.

As illustrated in FIG. 8 , a hole for fixation with the fixed shaft ofthe ferroelectric storage apparatus 100 is provided on one end side ofthe actuator arm 21, and the suspension arm 22 is connected to the tipend side of the actuator arm 21.

As illustrated in FIG. 8 , the actuator arm 21 is connected to one endside of the suspension arm 22, and the probe slider 23 is attached tothe tip of the suspension arm 22.

(Probe Slider)

As illustrated in FIG. 8 , the probe slider 23 is provided at the tipend of the suspension arm 22.

FIG. 10 is a cross-sectional view illustrating an example ofconfiguration of the probe slider 23. Of the arrows in FIG. 10 , a +Xaxis direction and a −X axis direction are sector directions of theferroelectric recording medium 10, a +Y axis direction and a −Y axisdirection are track directions of the ferroelectric recording medium 10,and a Z axis direction is a direction to face the recording surface ofthe ferroelectric recording medium 10. As illustrated in FIG. 10 , theconductive probe 26 is provided at the tip portion of the probe slider23.

FIG. 11 is a cross-sectional view illustrating another example ofconfiguration of the probe slider 23. The directions of arrows in FIG.11 are similar to the directions of arrows in FIG. 10 .

As illustrated in FIG. 11 , the conductive probe 26 and a piezoelectricelement 27 disposed between the probe slider 23 and the conductive probe26 may be provided at the tip portion of the probe slider 23. Thepiezoelectric element 27 includes a first piezoelectric element 27A anda second piezoelectric element 27B. The first piezoelectric element 27Adrives the conductive probe 26 in the +Z axis direction and the −Z axisdirection, and the second piezoelectric element 27B drives theconductive probe 26 in the +Y axis direction and the −Y axis direction.The piezoelectric element 27 may include not only the firstpiezoelectric element 27A and the second piezoelectric element 27B butalso a third piezoelectric element 27C and a fourth piezoelectricelement 27D, explained later.

FIG. 12 is a partially enlarged cross-sectional view of FIG. 11 . FIG.13 is a partially enlarged cross-sectional view of FIG. 11 as seen fromanother direction. FIG. 14 is a partially enlarged view as seen from thelower side of FIG. 11 . As illustrated in FIG. 12 to FIG. 14 , anelectrode 28A-1 is provided between the conductive probe 26 and thefirst piezoelectric element 27A, and an electrode 28A-2 is providedbetween the first piezoelectric element 27A and the second piezoelectricelement 27B. The electrode 28A-1 and the electrode 28A-2 disposed on theupper and lower surfaces of the first piezoelectric element 27A in the Zaxis direction constitute a pair to sandwich the first piezoelectricelement 27A.

The second piezoelectric element 27B is provided between the probeslider 23 and the electrode 28A-2, and an electrode 28B-1 and anelectrode 28B-2 are provided on side surfaces of the secondpiezoelectric element 27B in the Y axis direction. The electrode 28B-1is provided on a surface of the second piezoelectric element 27B on the+Y axis direction side, the electrode 28B-2 is provided on a surface ofthe second piezoelectric element 27B on the −Y axis direction side, andthe electrode 28B-1 and the electrode 28B-2 constitute a pair tosandwich the second piezoelectric element 27B.

A wire L11 is connected to the electrode 28A-2, a wire L12 is connectedto the electrode 28B-1, a wire L21 is connected to the electrode 28B-2,and a wire L22 is connected to the electrode 28A-1. The electrodes28A-1, 28A-2, 28B-1, and 28B-2 are connected via wires to an electrodepad 28C provided on the lower surface of the probe slider 23, theelectrode pad 28C being provided on the outer side of the electrode28A-2 in the Y axis direction.

In this case, the electrode 28A-2 is used to polarize the firstpiezoelectric element 27A, but, on the other hand, the electrode 28A-2is also in contact with the second piezoelectric element 27B, andtherefore, the electrode 28A-2 may polarize the second piezoelectricelement 27B. In order to prevent this, an insulating layer of a lowdielectric constant is preferably provided between the electrode 28A-2and the second piezoelectric element 27B, and the dielectric constant inthis case is preferably equal to or less than 1/100 of the dielectricconstant of the second piezoelectric element 27B.

The piezoelectric effect includes a piezoelectric vertical effect, apiezoelectric horizontal effect, and a piezoelectric thickness-sheareffect. At the electrode position as illustrated in FIG. 12 and FIG. 13, the piezoelectric vertical effect is used with respect to the firstpiezoelectric element 27A and the second piezoelectric element 27B, butin a case where other effects are used, electrodes may be provided onother surfaces of the piezoelectric element.

When the first piezoelectric element 27A is driven in the +Z axisdirection or the −Z axis direction, the first piezoelectric element 27Ais also displaced by the piezoelectric horizontal effect in the +Y axisdirection or the −Y axis direction. The displacement of the firstpiezoelectric element 27A in the +Y axis direction or the −Y axisdirection is preferably compensated by driving the second piezoelectricelement 27B in the +Y axis direction or the −Y axis direction.

(Conductive Probe)

The conductive probe 26 has a function of recording information to andreproducing information from the ferroelectric recording layer 13 of theferroelectric recording medium 10. The conductive probe 26 is aneedle-shaped conductive electrode used for a scanning probe microscopesuch as a scanning tunneling microscope (STM), an atomic forcemicroscope (AFM), and the like.

As illustrated in FIG. 11 , the conductive probe 26 is provided on thelower surface of the second piezoelectric element 27B via the electrode28A-1, and is preferably provided at a position shifted in the trackdirection of the ferroelectric recording medium 10 (the +Y axisdirection or the −Y axis direction) with reference to the center of thelower surface of the second piezoelectric element 27B. For example, asillustrated in FIG. 14 , the conductive probe 26 is preferably providedon the side of the electrode 28B-2 located on the +Y axis direction withreference to the center of the lower surface of the second piezoelectricelement 27B. Alternatively, conversely to FIG. 14 , the conductive probe26 may be provided on the side of the electrode 28B-1 located on the −Yaxis direction side with reference to the center of the lower surface ofthe second piezoelectric element 27B.

The conductive probe 26 travels by floating above the surface of theferroelectric recording medium 10. In recording of information to andreproducing of information from the ferroelectric recording medium 10,it is preferable that the recording medium is rotated at a high speedsimilar to an HDD, and the probe slider 23 is caused to float by an airflow caused on the recording medium surface due to the rotation, so thatinformation is recorded and reproduced with the conductive probe 26attached to the probe slider 23. Specifically, the conductive probe 26in the needle shape is attached to the probe slider 23, and this probeslider 23 is caused to travel by floating above the surface of theferroelectric recording medium 10 in an order of nanometers, so thatinformation is recorded and reproduced by bringing the conductive probe26 in the needle shape to a position extremely close to the surface ofthe ferroelectric recording medium 10.

Examples of materials constituting the conductive probe 26 includemetals such as tungsten, molybdenum, and platinum.

FIG. 15 is a cross-sectional view illustrating a configuration of theconductive probe 26. As illustrated in FIG. 15 for example, theconductive probe 26 includes: a base body 261 provided on thepiezoelectric element 27; and a needle-shaped electrode 262 formed onthe base body 261 and made into a sharp shape. The base body 261 and theneedle-shaped electrode 262 may be integrally formed of the samematerial, or may be formed of different materials.

Instead of the piezoelectric element 27, an electrostrictive element maybe used. Both of the piezoelectric element and the electrostrictiveelement have the same function in that a displacement occurs in responseto an electric field applied, but are different in that the direction ofdisplacement of the piezoelectric element changes depending on thedirection of the electric field, whereas the electrostrictive elementonly extends and does not shrink. Furthermore, they are different inthat the piezoelectric element generates a charge according to thestress, whereas the electrostrictive element does not generate a chargein response to stress applied. Because both of the piezoelectric elementand the electrostrictive element have the same function in that adisplacement occurs in response to an electric field applied, both ofthe piezoelectric element and the electrostrictive element can be usedfor the ferroelectric storage apparatus in a similar manner.

The needle-shaped electrode 262 is an electrode formed in a cone shape,and a width direction and a height of the needle-shaped electrode 262is, for example, several nanometers to millimeters.

When the curvature of the tip of the needle-shaped electrode 262decreases, the electric field strength at the tip increases, and thevoltage applied to the needle-shaped electrode 262 can be decreased,which is advantageous during recording of information to theferroelectric recording medium 10. Furthermore, this makes it easier tobring the tip of the needle-shaped electrode 262 into proximity to theferroelectric recording medium 10, which is advantageous during readingof information. The curvature radius of the tip of the needle-shapedelectrode 262 is preferably several nanometers or less.

The conductive probe 26 scans on the surface of the ferroelectricrecording medium 10 (the recording surface). The conductive probe 26 isbrought to a position extremely close to the surface of theferroelectric recording medium 10 (the recording surface). Then, anelectric field that exceeds the coercive electric field of theferroelectric layer 131 is applied from the conductive probe 26, and thepolarization direction of the ferroelectric layer 131 located directlyunder the conductive probe 26 is inverted. The applied voltage is madeinto a pulse signal of which the level changes according to informationto be recorded, and while the voltage is applied via the conductiveprobe 26 to the ferroelectric recording medium 10, the position of theconductive probe 26 with reference to the ferroelectric recording medium10 is moved in a direction parallel to the surface of the ferroelectricrecording medium 10. Accordingly, information can be recorded bypolarizing the ferroelectric of the ferroelectric layer 131 of theferroelectric recording medium 10.

A method for reproducing information recorded in the ferroelectricrecording medium 10 is explained later.

The conductive probe 26 can be manufactured by using any givenmanufacturing method. For example, a manufacturing method of theconductive probe 26 may include a step of forming a mask in a dot shapeon the surface of the conductive material, a step of obtaining aneedle-shaped electrode formed in a cone shape by etching the conductivematerial, and a step of removing the mask. Accordingly, the conductiveprobe 26 is formed so that the conductive probe 26 is formed in a coneshape on the base body 261 made of the etched conductive material, andincludes the needle-shaped electrode 262 that is made into a sharpshape.

An example of the manufacturing method of the conductive probe 26 isexplained. FIG. 16 is a drawing illustrating an example of themanufacturing method of the conductive probe 26. As illustrated in FIG.16 , after the mask 71 in the dot shape is formed on the surface of theconductive material 260 (see FIG. 16 (a)), the conductive material. 260is etched (see FIG. 16 (b)). Etching is delayed in the portion of theconductive material 260 where the mask 71 is provided, and accordingly,as a result of lift off of the mask 71, the needle-shaped electrode 262in a substantially cone shape is formed under the mask 71 (see FIG. 16(c)). Thereafter, the conductive material 260 around the needle-shapedelectrode 262 is cut off, so that the conductive probe 26 including theneedle-shaped electrode 262 formed in a cone shape on the base body 261is formed (see FIG. 16 (d)).

In a case where a step of cutting out the needle-shaped electrode 262 isperformed by machine processing, the needle-shaped electrode 262 may bedamaged. In this case, it is preferable to etch the conductive material260, after the location where the needle-shaped electrode is to beformed is cut out.

In a case where the conductive material 260 used in the above-describedstep is an insulator or a semiconductor material, the conductive filmmay be formed by coating the surface of the manufactured needle-shapedelectrode with Au (gold) or the like by a sputtering method, so thatconductivity is imparted to the needle-shaped electrode.

Alternatively, the conductive probe 26 may be manufactured by usingother manufacturing methods. FIG. 17 is a drawing illustrating anexample of another manufacturing method of the conductive probe 26. Asillustrated in FIG. 17 , a photoresist 72 is applied to the surface ofthe conductive material 260 (a photoresist application step (see FIG. 17(a))).

Next, the photoresist 72 is etched in a circular shape, and a mask 72Ahaving a very small through hole (hole) 72 a in a circular shape isformed in the photoresist 72 (a mask formation step (see FIG. 17 (b))).

Next, a metal 73 is deposited onto the surface of the conductivematerial 260 in a through hole 72 a and onto the mask 72A to form theneedle-shaped electrode 262 formed in the substantially cone shape (aformation step of the needle-shaped electrode (see FIGS. 17 (c) and(d))).

In this case, when the amount of deposited metal 73 that is formed bydeposition on the mask 72A increases, the through hole 72 a is closedand the width of the through hole 72 a decreases. Accordingly, the metal73 deposited on the bottom portion in the through hole 72 a of the mask72A is widely deposited on the bottom portion, and the deposition rangegradually decreases toward the upper side. Ultimately, the needle-shapedelectrode 262 deposited in the substantially cone shape is formed (seeFIG. 17 (d)).

Next, the conductive probe 26 including the needle-shaped electrode 262is obtained by removing the mask 72A (a production step of theconductive probe (see FIG. 17 (e))). The conductive probe 26 includingthe needle-shaped electrode 262 formed in the cone shape on the basebody 261 is obtained by removing the mask 72A and cutting off theconductive material 260 around the needle-shaped electrode 262.

The conductive probe 26 can be used in various aspects. Aspects of theconductive probe 26 are explained.

((First Aspect))

As illustrated in FIG. 18 and FIG. 19 , a conductive probe 26A ispreferably formed in a triangular or quadrangular pyramid. Whenanisotropic etching is performed on the conductive material usingsingle-crystal, a top portion formed by any given crystal plane (atextured surface) can be readily formed.

For example, in a case where the conductive material 260 uses a (100)plane of single-crystal having a diamond structure such as Si, the (100)plane is processed as the crystal plane, so that the conductive probe26A can be readily formed in a quadrangular pyramid that includes four(111) equivalent planes and that includes a sharp top portion.Therefore, in a case where the conductive probe 26A is in asubstantially triangular pyramid or substantially quadrangular pyramidshape, the conductive probe 26A may be made into a conductive probehaving a sharp top portion, as compared with the case where theconductive probe 26A is in a substantially cone shape.

In a case where the conductive probe 26A is in substantially triangularpyramid or substantially quadrangular pyramid shape, the conductiveprobe 26A can have a sharper top portion, as compared with the casewhere the conductive probe 26A is in a substantially cone shape similarto the conductive probe 26.

Furthermore, in order to uniformize the electric field distribution thatoccurs at the tip of the conductive probe 26A during writing and tostabilize the tunnel current flowing between the conductive probe 26Aand the ferroelectric layer 131 during reading, the conductive probe 26Ais preferably rotationally symmetric about the axis that passes the tipthereof.

The conductive probe 26A can be manufactured using any givenmanufacturing method. A manufacturing method of the conductive probe 26Amay be such that, for example, the shape of the mask 71 formed on thesurface of the conductive material 260 is a triangular or quadrangularshape in the manufacturing method of the conductive probe as illustratedin FIG. 16 explained above. Accordingly, as illustrated in FIG. 18 andFIG. 19 , the conductive probe 26A including the needle-shapedelectrode. 262A in a triangular pyramid or quadrangular pyramid shape onthe base body 261 can be formed.

It is preferable to perform anisotropic etching on the conductivematerial 260 using single-crystal, and use a method for forming the topportion formed by any given crystal plane (textured surface). Forexample, in a case where the conductive material 260 uses the (100)plane of single-crystal having a diamond structure such as Si, theconductive probe 26A can form, with a high reproducibility, thequadrangular pyramid that is constituted by four (111) equivalent planesand that includes a sharp top portion.

In this case, for the anisotropic etching, it is preferable to usereactive ion etching (RIE) using an etching gas such as SF₆, wet etchingusing KOH as an etchant, and the like.

Alternatively, the conductive probe 26A can be manufactured using othermanufacturing methods. Another manufacturing method of the conductiveprobe 26A may be such that, for example, the shape of the through hole72 a formed in the photoresist 72 is a triangular or quadrangular shape,and the mask 72A including the through hole 72 a in the triangular orquadrangular shape in the photoresist 72 is formed, in the maskformation step in the manufacturing method of the conductive probe asillustrated in FIG. 17 explained above. The shape of the through hole 72a is formed in the triangular or quadrangular shape, so that theneedle-shaped electrode 262 in the substantially triangular pyramid orquadrangular pyramid shape is formed on the conductive material 260.Accordingly, as illustrated in FIG. 18 and FIG. 19 , the conductiveprobe 26A having the needle-shaped electrode 262A in the triangularpyramid or quadrangular pyramid shape on the base body 261 is obtained.

((Second Aspect))

FIG. 20 illustrates a cross-sectional view of another configuration ofthe conductive probe 26. As illustrated in FIG. 20 , a conductive probe26B preferably includes a base body 261 constituted by a conductivematerial, a recessed portion 261 a formed in the base body 261, and aneedle-shaped electrode 262 formed in a cone shape in the recessedportion 261 a, wherein a portion of the needle-shaped electrode 262preferably protrudes from a surface (principal surface) 261 b of thebase body 261. The surface 261 b of the base body 261 refers to asurface of the principal surface of the base body 261 that is other thanthe recessed portion 261 a.

In the conductive probe 26B, most of the needle-shaped electrode 262 issurrounded by and covered with the base body 261, and therefore, whenthe conductive probe 26B accidentally comes into contact withferroelectric recording medium 10, damage to the needle-shaped electrode262 can be alleviated. Furthermore, due to air flow caused by rotationof the ferroelectric recording medium 10, occurrence of vibration anddeformation of the needle-shaped electrode 262 can be alleviated.

FIG. 21 is a cross-sectional view illustrating an example ofconfiguration of the probe slider 23 on which the conductive probe 26Bis provided.

A manufacturing method of the conductive probe 26B is explained. Themanufacturing method of the conductive probe 26B includes a step ofapplying a photoresist to the surface of the conductive material, a stepof forming a mask having a very small through hole (hole) by patterninga photoresist, a step of forming a recessed portion in the surface ofthe conductive material by etching the conductive material in the hole,a step of depositing a metal film on a bottom portion of the recessedportion of the conductive material formed in the through hole, and astep of obtaining a needle-shaped electrode formed in a cone shape onthe bottom portion of the recessed portion of the conductive material byremoving the photoresist, wherein a portion of the needle-shapedelectrode 262 is caused to protrude from the conductive material.According to the above, the conductive probe 26B is obtained.

FIG. 22 is an explanatory diagram illustrating an example of themanufacturing method of the conductive probe 26B. As illustrated in FIG.22 , after a photoresist 82 is applied to the surface of the conductivematerial 260 (see FIG. 22 (a)) and patterning is performed, a mask 82Aincluding a very small through hole (hole) 82 a in the photoresist 82 isformed (see FIG. 22 (b)). Thereafter, a recessed portion 260 a is formedin the surface of the conductive material 260 by etching the throughhole 82 a using any given etching method as appropriate (see FIG. 22(c)). After the etching, a metal 83 is deposited onto the bottom portionof the recessed portion 260 a of the conductive material 260 formed inthe through hole 82 a and onto the photoresist 82 (see FIG. 22 (d)). Inthis case, when the amount of deposited metal 83 that is formed bydeposition on the photoresist 82 increases, the through hole 82 a isclosed and the width of the through hole 82 a decreases. Accordingly,the metal 83 deposited on the bottom portion in the recessed portion 260a of the conductive material 260 is widely deposited on the bottomportion, and the deposition range gradually decreases toward the upperside. Ultimately, the needle-shaped electrode 262 deposited in the coneshape is formed (see FIG. 22 (e)).

Thereafter, by removing the mask 82A (see FIG. 22 (f)), a portionincluding the top portion of the needle-shaped electrode 262 in the coneshape is caused to slightly protrude from the conductive material 260.

By cutting off the conductive material 260 around the needle-shapedelectrode 262, the conductive probe 26B including the needle-shapedelectrode 262 formed in the cone shape is formed in the recessed portion261 a of the base body 261 as illustrated in FIG. 20 .

According to this manufacturing method, the needle-shaped electrode 262is formed in the recessed portion 261 a, and is surrounded by andcovered with the base body 261, and therefore, when the conductive probe26B accidentally comes into contact with the ferroelectric recordingmedium 10, damage to the needle-shaped electrode 262 can be alleviated.Furthermore, due to air flow caused by rotation of the ferroelectricrecording medium 10, occurrence of vibration and deformation of theneedle-shaped electrode 262 can be alleviated.

When the shape of the through hole 82 a is made into a triangular orquadrangular shape, the conductive probe 26B can be formed in atriangular pyramid or quadrangular pyramid shape.

((Third Aspect))

FIG. 23 illustrates an example of configuration of the conductive probe26. As illustrated in FIG. 23 , a conductive probe 26C preferablyincludes an insulating layer 263, formed by oxidizing the conductivematerial 260 through heating, on the base body 261 constituted by theconductive material, wherein the needle-shaped electrode 262 is providedon the base body 261 in a through hole 263 a of the insulating layer263, and a portion of the needle-shaped electrode 262 protrudes from asurface (principal surface) 263 b of the insulating layer 263. Thesurface 263 b of the insulating layer 263 refers to a surface of theprincipal surface of the base body 261 that is other than the throughhole 263 a.

The needle-shaped electrode 262 is formed on the surface of the basebody 261 in the through hole 263 a, and is surrounded by and coveredwith the insulating layer 263, and therefore, when the needle-shapedelectrode 262 accidentally comes into contact with the ferroelectricrecording medium 10, damage to the needle-shaped electrode 262 can bealleviated. Furthermore, due to air flow caused by rotation of theferroelectric recording medium 10, occurrence of vibration anddeformation of the needle-shaped electrode 262 can be alleviated. Stillfurthermore, because the insulating layer 263 is provided around theneedle-shaped electrode 262, the needle-shaped electrode 262 isshielded, so that the influence caused by charge around theneedle-shaped electrode 262 and leakage of the charge in theneedle-shaped electrode 262 to the outside can be alleviated.

A manufacturing method of the conductive probe 26C is explained. Themanufacturing method of the conductive probe 26C includes a step offorming, on the conductive material 260, the insulating layer byoxidizing the conductive material 260 and a step of forming an isolationlayer on the insulating layer, wherein a portion of the needle-shapedelectrode is caused to protrude from the insulating layer. As a result,the conductive probe 26C is obtained.

FIG. 24 is an explanatory diagram illustrating an example of themanufacturing method of the conductive probe 26C. As illustrated in FIG.24 , the insulating layer 263 is formed by heating and oxidizing theconductive material 260 (see FIGS. 24 (a) and (b)). Thereafter, theisolation layer 29 is formed on the insulating layer 263, thephotoresist 82 is applied to the surface of the isolation layer 29 andpatterning is performed, and a very small through hole 82 a in acircular shape is formed in the photoresist 82 (see FIG. 24 (c)).Thereafter, the insulating layer 263 and the isolation layer 29 of thethrough hole 82 a are etched using any given etching method asappropriate (see FIG. 24 (d)). In FIG. 24 (d), the photoresist 82 isremoved after the insulating layer 263 and the isolation layer 29 areetched, but at least a portion of the photoresist 82 may be preserved.

After the etching, the metal 83 is deposited onto the surface of theconductive material 260 in the through holes 263 a and 29 a of theinsulating layer 263 and the isolation layer 29, respectively, and ontothe isolation layer 29 (see FIG. 24 (e)). In this case, when the amountof deposited metal 83 that is formed by deposition on the photoresist 82increases, the through hole 29 a is closed and the width of the throughhole 29 a decreases. Accordingly, the metal 83 deposited on the surfaceof the conductive material 260 in the through hole 29 a is widelydeposited on the surface, and the deposition range gradually decreasestoward the upper side. Ultimately, the metal 83 is deposited in the coneshape. Accordingly, the needle-shaped electrode 262 deposited in thecone shape is formed.

Thereafter, by etching the isolation layer 29, a portion including thetop portion of the needle-shaped electrode 262 in the cone shape iscaused to slightly protrude from the principal surface of the insulatinglayer 263 (see FIG. 24 (f)).

By cutting off the conductive material 260 around the needle-shapedelectrode 262, the conductive probe 26C including the needle-shapedelectrode 262 formed in the cone shape is formed in the through hole 263a of the insulating layer 263 on the base body 261 as illustrated inFIG. 23 .

According to this manufacturing method, the needle-shaped electrode 262is formed on the surface of the base body 261 in the through hole 263 a,and is surrounded by and covered with the insulating layer 263, andtherefore, when the conductive probe 26C accidentally comes into contactwith the ferroelectric recording medium 10, damage to the needle-shapedelectrode 262 can be alleviated. Furthermore, due to air flow caused byrotation of the ferroelectric recording medium 10, occurrence ofvibration and deformation of the needle-shaped electrode 262 can bealleviated. Still furthermore, because the insulating layer 263 isprovided around the needle-shaped electrode 262 in the conductive probe26C, the needle-shaped electrode 262 is shielded, so that the influencecaused by charge around the needle-shaped electrode 262 and leakage ofthe charge in the needle-shaped electrode. 262 can be alleviated byshielding the needle-shaped electrode 262.

(First Piezoelectric Element and Second Piezoelectric Element)

As illustrated in FIG. 11 , in the probe slider 23 having theconfiguration as illustrated in FIG. 11 , the first piezoelectricelement 27A is provided, at the tip portion of the probe slider 23,between the probe slider 23 and the conductive probe 26, so that thefirst piezoelectric element 27A is sandwiched between the electrode28A-1 and the electrode 28A-2. Also, the second piezoelectric element27B is provided between the tip portion of the probe slider 23 and theelectrode 28A-2, and the electrode 28B-1 and the electrode 28B-2 areprovided on the side surface of the second piezoelectric element 27B onthe +Y axis direction side and the side surface of the secondpiezoelectric element 27B on the −Y axis direction side, respectively.The first piezoelectric element 27A can be used to adjust the height offloating of the header (dynamic fly height (DFH)) of the conductiveprobe 26, and the second piezoelectric element 27B has a function ofmoving the conductive probe 26 in the track direction of theferroelectric recording medium 10.

In the present embodiment, electrostrictive elements may be used inplace of the first piezoelectric element 27A and the secondpiezoelectric element 27B. Both of the first piezoelectric element 27Aand the second piezoelectric element 27B and the electrostrictiveelement have the same function in that a displacement occurs in responseto an electric field applied, but are different in that the direction ofdisplacement of the first piezoelectric element 27A and the secondpiezoelectric element 27B change depending on the direction of theelectric field, whereas the electrostrictive element only extends anddoes not shrink. Furthermore, they are different in that the firstpiezoelectric element 27A and the second piezoelectric element 27Bgenerate charge according to the stress, whereas the electrostrictiveelement does not generate a charge in response to stress applied.Because both of the first piezoelectric element 27A and the secondpiezoelectric element 27B and the electrostrictive element have the samefunction in that a displacement occurs in response to an electric fieldapplied, both of the first piezoelectric element 27A and the secondpiezoelectric element 27B and the electrostrictive element can be usedfor the ferroelectric storage apparatus in a similar manner.

As illustrated in FIG. 11 , the first piezoelectric element 27A isprovided on the surface of the lower end side at the tip of the probeslider 23, and can be used for adjustment of the height of floating ofthe header (dynamic fly height (DFH)) of the conductive probe 26.Specifically, the first piezoelectric element 27A is provided betweenthe probe slider 23 and the conductive probe 26, and the distancebetween the ferroelectric recording medium 10 and the conductive probe26 is adjusted, so that the probe slider 23 can be caused to travel byfloating above the surface of the ferroelectric recording medium 10 by adistance in an order of nanometers. The amount of displacement of thefirst piezoelectric element 27A can be controlled in an order ofnanometers, and the responsiveness thereof is 10 microseconds or less.Therefore, the distance between the conductive probe 26 and theferroelectric recording medium 10 can be controlled at a high speed witha high degree of accuracy.

The first piezoelectric element 27A and the second piezoelectric element27B may be constituted by, for example, quartz, lithium niobate(LiNbO₃), barium titanate (BaTiO₃), titanate lead zirconate (PZT), zincoxide (ZnO), aluminum nitride (AlN), lithium tantalate (LiTaO₃), leadtitanate (PT), and the like. In a case where electrostrictive elementsare used in place of the first piezoelectric element 27A and the secondpiezoelectric element 27B, the electrostrictive elements may beconstituted by materials listed above.

The first piezoelectric element 27A is provided at the tip portion ofthe probe slider 23, so that the DFH control of the conductive probe 26can be performed at a high speed with a high degree of accuracy, and thedistance between the conductive probe 26 and the ferroelectric recordingmedium 10 can be controlled at a high speed with a high degree ofaccuracy. Therefore, the ferroelectric storage apparatus 100 can furtherenhance the recording and reproduction sensitivity of the conductiveprobe 26.

Furthermore, because the first piezoelectric element 27A is provided atthe tip portion of the probe slider 23, the probe slider 23 can have anautomatic gain control (AGC) function between tracks and between sectorsof the ferroelectric recording medium 10.

For example, as explained later, a voice coil motor can be used as theprobe driving unit 40 (see FIG. 8 ) that moves the conductive probe 26in the track direction of the ferroelectric recording medium 10.However, in a case where a voice coil motor is used, the accuracy ofpositioning of the conductive probe 26 is about 10 nm. Furthermore, ittakes several milliseconds for moving time to another track (seek time),which becomes an obstacle in increasing the capacity and speed of theferroelectric storage apparatus 100.

In the present embodiment, the second piezoelectric element 27B is usedto move the conductive probe 26 in the track direction of theferroelectric recording medium 10, and therefore, the accuracy ofpositioning in the track direction of the ferroelectric recording medium10 can be 1 nm or less, and a correcting operation in a same track and amoving operation to another track can be performed in several microseconds or less. Accordingly, the capacity and the speed of theferroelectric storage apparatus 100 can be increased.

As illustrated in FIG. 12 and FIG. 13 , the first piezoelectric element27A and the second piezoelectric element 27B are provided between theprobe slider 23 and the conductive probe 26. Because the firstpiezoelectric element 27A and the second piezoelectric element 27B areprovided stably between the probe slider 23 and the conductive probe 26,the first piezoelectric element 27A and the second piezoelectric element27B are preferably formed as a columnar body such as a cube, arectangular parallelepiped, a cylinder, and the like. The pair ofelectrodes 28A-1 and 28A-2 are provided on the surface of the firstpiezoelectric element 27A on the +Z axis direction side and the surfaceof the first piezoelectric element 27A on the −Z axis direction side.The electrode 28A-1 is provided between the conductive probe 26 and thefirst piezoelectric element 27A. The electrode 28A-2 is provided betweenthe first piezoelectric element 27A and the second piezoelectric element27B.

As illustrated in FIG. 14 , the pair of electrodes 28B-1 and 28B-2 isprovided on the second piezoelectric element 27B. The electrode 28B-1 isprovided on the −Y axis direction side of the second piezoelectricelement 27B. The electrode 28B-2 is provided on the +Y axis directionside of the second piezoelectric element 27B.

The electrodes 28A-1 and 28A-2 cause the first piezoelectric element 27Ato expand and contract in the +Z axis direction and the −Z axisdirection, and are used to apply a voltage for adjusting the distancebetween the conductive probe 26 and the ferroelectric recording medium10. This feature is explained later.

The electrodes 28B-1 and 28B-2 cause the second piezoelectric element27B to expand and shrink in the +Y axis direction and the −Y axisdirection, and are used to move the conductive probe 26 provided on theprobe slider 23 in the track direction of the ferroelectric recordingmedium 10. The surface of the second piezoelectric element 27B on the −Zaxis direction side is fixed to the probe slider 23, and the surface ofthe second piezoelectric element 27B on the +Z axis direction is notfixed. Accordingly, when a voltage is applied to the electrodes 28B-1and 28B-2, a surface 271 of the second piezoelectric element 27B on the+Z axis direction side expands and shrinks in the +Y axis direction andthe −Y axis direction. In this case, the surface 271 of the secondpiezoelectric element 27B on the +Z axis direction side expands andshrinks uniformly in the +Y axis direction and the −Y axis direction,and accordingly, the center of the surface is not displaced in the Yaxis direction, but the conductive probe 26 is provided with a shift inposition from the center of the surface 271 of the second piezoelectricelement 27B on the +Z axis direction side, and accordingly, theconductive probe 26 is displaced in the +Y axis direction or the −Y axisdirection that is the track direction of the ferroelectric recordingmedium 10. For example, as illustrated in FIG. 25 , in a case where thesecond piezoelectric element 27B expands in the +Y axis direction and inthe −Y axis direction, the conductive probe 26 is displaced in the +Yaxis direction indicated by the arrow.

In moving the conductive probe 26 in the track direction of theferroelectric recording medium 10, it is preferable to move theconductive probe 26 by expanding and shrinking the second piezoelectricelement 27B by using the probe driving unit 40 such as a voice coilmotor, a pulse motor, and the like in a rough operation in which themovement distance of the conductive probe is equal to or more than 10 nmand by using the second piezoelectric element 27B provided on the probeslider 23 in a fine operation in which the movement distance of theconductive probe 26 is less than 10 nm.

[Ferroelectric Recording Medium Driving Unit]

As illustrated in FIG. 8 , the ferroelectric recording medium drivingunit 30 drives and rotates the ferroelectric recording medium 10. FIG.26 is a cross-sectional view illustrating a configuration of theferroelectric recording medium driving unit 30. As illustrated in FIG.26 , the ferroelectric recording medium driving unit 30 includes ahousing (bearing cylinder) 31, a bearing sleeve 32, a shaft member(spindle shaft) 33, a housing bottom portion 34, permanent magnets 35, astator 36, and lubricant oil O. The ferroelectric recording mediumdriving unit 30 supports, in a housing 31, the spindle shaft 33 in theradial direction of the spindle shaft 33 (the direction orthogonal tothe spindle shaft 33) in a non-contact state, with the pressuregenerated by a dynamic pressure action of the lubricant oil O.

The housing 31 is a container for containing a portion of the spindleshaft 33, and is formed so that the spindle shaft 33 can be insertedthereto.

The bearing sleeve 32 is provided in the housing 31.

The spindle shaft 33 is a stick-shaped member inserted to the innercircumferential surface of the bearing sleeve 32, and has aconductivity. The spindle shaft 33 can be formed by a conductivematerial such as metal and the like. The spindle shaft 33 is insertedinto the opening portion 10 a of the ferroelectric recording medium 10(see FIG. 3 ), so that the spindle shaft 33 is connected via thesubstrate 11 and the electrode layer 12 to the ferroelectric recordinglayer 13 (see FIG. 3 ). Because the spindle shaft 33 has conductivity,information can be read from and information can be written to theferroelectric recording layer 13 with the conductive probe 26.

The shaft end portion 331 of the spindle shaft 33 preferably has acurved surface formed in a convex shape. A curvature radius of thecurved surface of the shaft end portion 331 is preferably 2 mm or moreand is more preferably 5 mm or more. Note that the curved surface of theshaft end portion 331 may be formed in a concave shape.

Groove portions 332 in a V shape can be provided on the outercircumference of the spindle shaft 33. The spindle shaft 33 includes thegroove portions 332, so that when the spindle shaft 33 rotates, a flowoccurs in the lubricant oil O, and the lubricant oil O can be readilycollected at the vertices of the V shapes of the groove portions 332.Therefore, a pressure is generated, and the spindle shaft 33 issupported.

The housing bottom portion 34 is provided in the housing 31 to face theshaft end portion 331 of the spindle shaft 33, and has conductivity.

Similar to the shaft end portion 331, the housing bottom portion 34 alsopreferably includes a curved surface formed in a convex shape or aconcave shape. A curvature radius of the curved surface of the housingbottom portion 34 is preferably 2 mm or more and is more preferably 5 mmor more.

The multiple permanent magnets 35 are provided on the innercircumferential surface of the cover 37 along the circumferentialdirection.

The stator 36 is provided between the housing 31 and the permanentmagnet 35.

The lubricant oil O fills the gap between the inner circumferentialsurface of the bearing sleeve 32 and the outer circumference surface ofthe spindle shaft 33. The lubricant oil O supports the spindle shaft 33in the radial direction thereof in a non-contact state by generating apressure with a dynamic pressure action.

The lubricant oil O preferably includes conductive powder of aninorganic matter. When the lubricant oil O includes the conductivepowder of the inorganic matter, the housing 31 and the spindle shaft 33can be brought into conduction with each other.

The conductive powder of the inorganic matter is preferably metallicpowder such as silver, copper, nickel, tin, silver-plated copper,stainless steel, aluminum, brass, iron, and zinc; carbon-based powdersuch as carbon, carbon black, graphite, carbon nanotube, and the like;metal oxide-based powder such as tin oxide, indium oxide, zinc oxide,and the like; and a metal-plated material by forming a coating layer ona surface of glass, mica powder, glass fiber, carbon fiber, and thelike.

The form of the metallic powder is preferably a powder form, a sphericalform, a fibrous form, or a foil piece.

The form of the carbon-based powder is preferably a spherical form or afibrous form.

The form of the metal oxide-based powder is preferably a powder form ora spherical form.

The form of the metal-plated material-based powder is preferably apowder form or a spherical form.

These materials have a high heat resistance and volatility resistance,and do not increase the viscosity of the lubricant oil O even when thematerials are contained in the lubricant oil O.

In a case where the conductive powder is a particle shape, the particlediameter of the conductive powder is preferably 0.1 μm to 10 μm. In acase where the conductive powder is the foil piece, one side of theconductive powder is preferably 0.1 μm to 100 μm. In a case where theconductive powder is fiber, the length of the fiber is preferably 0.1 μmto 100 μm. In a case where the conductive powder is a particle shape, afoil piece, or fiber, where the particle diameter of the conductivepowder is in the above-described preferred range, the conductive powdercan be distributed in the lubricant oil O without increasing theviscosity of the lubricant oil O.

In the electric recording medium driving unit 30, the spindle shaft 33is rotated by the attraction between the permanent magnet 35 and theelectromagnet of the stator 36. A thrust toward the lower side in FIG.26 is applied to the spindle shaft 33 by a method using magneticattraction with the electromagnet of the stator 36, a method using theweight, or other methods, and this thrust is supported by the housingbottom portion 34.

[Probe Driving Unit]

As illustrated in FIG. 8 , the probe driving unit 40 drives the probeslider 23.

[Control Unit]

As illustrated in FIG. 11 , in a case where the first piezoelectricelement 27A and the second piezoelectric element 27B are provided,between the probe slider 23 and the conductive probe 26, at the tipportion of the probe slider 23, the ferroelectric storage apparatus 100may include a control unit, not illustrated.

A control unit, not illustrated, is attached, as a printed circuit board(PCB), on the back side of the housing 60. The control unit, notillustrated, is electrically connected to the first piezoelectricelement 27A, the second piezoelectric element 27B, and the conductiveprobe 26. The control unit, not illustrated, has a function ofcontrolling the voltage applied to the first piezoelectric element 27Aand the second piezoelectric element 27B to cause the firstpiezoelectric element 27A and the second piezoelectric element 27B toexpand and shrink, and adjusting the distance and relative position ofthe ferroelectric recording medium 10 and the conductive probe 26. Thecontrol unit, not illustrated, preferably controls the voltage appliedto the first piezoelectric element 27A and the second piezoelectricelement 27B to cause the first piezoelectric element 27A and the secondpiezoelectric element 27B to expand and shrink, on the basis of the readsignal from the conductive probe 26.

When the first piezoelectric element 27A expands and shrinks in the +Zaxis direction or the −Z axis direction, the first piezoelectric element27A expands and shrinks in the +Y axis direction or the −Y axisdirection. The control unit, not illustrated, preferably has a functionof compensating expansion and shrinkage of the first piezoelectricelement 27A in the +Y axis direction or the −Y axis direction by causingthe second piezoelectric element 27B to expand and shrink in the +Y axisdirection or the −Y axis direction.

In a case where the read signal level from the conductive probe 26 islow, the control unit, not illustrated, increases the signal level byreducing the distance between the conductive probe 26 and theferroelectric recording medium 10 by increasing the voltage applied tothe first piezoelectric element 27A. Conversely, in a case where theread signal level from the conductive probe 26 is high, the controlunit, not illustrated, decreases the signal level by increasing thedistance between the conductive probe 26 and the ferroelectric recordingmedium 10 by decreasing the voltage applied to the first piezoelectricelement 27A. Accordingly, the distance between the ferroelectricrecording medium 10 and the conductive probe 26A can be controlled witha high degree of accuracy and response. Therefore, the control unit, notillustrated, allows the ferroelectric storage apparatus 100 to have anAGC (automatic gain control) function between tracks and between sectorsof the ferroelectric recording medium 10.

[Recording-and-Reproduction Signal Processing Unit]

The recording-and-reproduction signal processing unit 50 as illustratedin FIG. 8 has a function of performing processing of write and readsignals of information with respect to the conductive probe 26.

The recording-and-reproduction signal processing unit 50 writes, byusing the conductive probe 26, information to the ferroelectricrecording layer 13 constituting the ferroelectric recording medium 10 byapplying a positive or negative voltage. In addition, therecording-and-reproduction signal processing unit 50 reads, by using theconductive probe 26, information by reading positive or negative chargewritten in the ferroelectric recording layer 13. During writing, therecording-and-reproduction signal processing unit 50 generates apositive or negative voltage, corresponding to write information, thatis to be applied to the conductive probe 26. During reading, therecording-and-reproduction signal processing unit 50 converts anelectric signal from the conductive probe 26 into written information byprocessing the electric signal.

The recording-and-reproduction signal processing unit 50 includes abipolar power source, not illustrated. During writing, for example, in acase where information to be written is a binary number of either 1 or0, the recording-and-reproduction signal processing unit 50 uses thebipolar power source, not illustrated, to generate a positive voltagewhen the information to be written is one, and generate a negativevoltage when the information to be written is zero.

The recording-and-reproduction signal processing unit 50 preferablyrecords (writes) multi-value information to the ferroelectric layer 131included in the ferroelectric recording layer 13 of the ferroelectricrecording medium 10 and reproduces (reads) recorded multi-valueinformation. Recording and reproduction of multi-value information areas described above, and the details thereof are omitted.

As described above, in the ferroelectric layer 131, position information(servo information) for detecting relative position between theconductive probe 26 of the probe slider 23 and the track on theferroelectric recording medium 10 may be recorded. In the ferroelectriclayer 131, servo information areas recorded with the servo informationand areas for recording and reproducing data may be arranged alternatelywith regular intervals in the circumferential direction of theferroelectric recording medium 10.

The servo information may be written to the ferroelectric recordingmedium 10 by using a servo writer, not illustrated, before theferroelectric recording medium 10 is assembled into the ferroelectricstorage apparatus 100, or may be written using therecording-and-reproduction signal processing unit 50 after theferroelectric recording medium 10 is assembled into the ferroelectricstorage apparatus 100. In the latter case, after the actuator arm 21 orthe suspension arm 22 is mechanically fixed using a lever, notillustrated, from the outside of the ferroelectric storage apparatus100, servo information may be written to the ferroelectric recordingmedium 10 while the conductive probe 26 is positioned on the surface ofthe ferroelectric recording medium 10 by slightly moving the lever, notillustrated.

In this case, servo information and data areas in which data is recordedand reproduced are preferably arranged alternately with regularintervals in the circumferential direction of the track on theferroelectric recording medium 10. Accordingly, during reproduction ofrecorded data, the probe slider 23 can more appropriately detect theposition of the conductive probe 26 with the servo information.

As described above, the recording-and-reproduction signal processingunit 50 preferably records multi-value information to the recording areaof the smallest size with a simplest single write operation by theferroelectric storage apparatus 100, and reproduces the informationmulti-value recorded in the ferroelectric recording medium 10 with asimplest single read operation by the ferroelectric storage apparatus100.

As described above, the servo information area 131B of the ferroelectriclayer 131 may include the burst information area 131B-1, the addressinformation area 131B-2, and the preamble information area 131B-3. Inthis case, the conductive probe 26 moving in the circumferentialdirection on the surface of the ferroelectric recording medium 10 readspreamble information from the preamble information area 131B-3 toprepare for reading address information. Then, the conductive probe 26reads the address information of the data area from the addressinformation area 131B-2. Then, the probe slider 23 performs fineadjustment of the track position (radius position) by reading burstinformation from the burst information area 131B-1. Thereafter, theconductive probe 26 can record information in the data area 131A.

As described above, the servo information area 131B of the ferroelectriclayer 131 may include the reference signal information 131B-4 ofmulti-value recording. In this case, the conductive probe 26 reads thereference signal information from the servo information area 131B, sothat the recording-and-reproduction signal processing unit 50 canascertain the signal levels of multi-value recording, and reproduce themulti-value information recorded in the data area 131A by using theascertained signal levels.

When the recording-and-reproduction signal processing unit 50 writesinformation to the ferroelectric recording medium 10, therecording-and-reproduction signal processing unit 50 preferably adjuststhe voltage waveform emitted from a bipolar power source, notillustrated, to be applied to the conductive probe 26, so that thevoltage waveform becomes any one of a triangle wave, sawtooth wave, andtrapezoidal wave, as illustrated in FIG. 27 . In a case where thevoltage waveform emitted from the bipolar power source, not illustratedis a square wave, a large amount of charge flows the moment the positiveor negative voltage is applied, and accordingly, the sharp needle tip ofthe conductive probe 26 may become dull due to thermal melting and rapidelectric field evaporation. In the present embodiment, the voltagewaveform is any one of a triangle wave, a sawtooth wave, and atrapezoidal wave, so that the potential can be increased gradually fromthe zero potential, and accordingly, damage to the conductive probe 26can be alleviated.

It is known that, during reading, information written to theferroelectric recording medium 10 can be read as a capacitive change,for example, by reading a capacitive change of the ferroelectric layer131 between the conductive probe 26 and the electrode layer 12 whileapplying, to the conductive probe 26, an alternating current electricfield smaller than the coercive electric field of the ferroelectriclayer 131. The principle of the above is as follows.

Where a voltage applied to the conductive probe 26 is denoted as E, anelectric flux density due to charge of the ferroelectric is denoted asD, a dielectric constant is denoted as ε, and a polarization voltage isdenoted as P, the electric flux density D due to charge of theferroelectric is expressed by the following expression (1).

$\begin{matrix}{D = {P + {\sum\limits_{n = 1}^{\infty}{\frac{ɛ(n)}{n!}E^{n}}}}} & (1)\end{matrix}$

In this case, in a case where the voltage E is an alternating current,the following expression (2) is satisfied, and when the expression (2)is substituted into the above expression (1), the dielectric constantsfor odd-number indexes, i.e., ε3, ε5, become non-linear, and the signchanges according to the direction of the spontaneous polarization ofthe ferroelectric. Therefore, the direction of the spontaneouspolarization of the ferroelectric can be determined by measuring thechange of the dielectric constant.E=E _(p) cos ωt,where E _(p) denotes the peak voltage of the alternating current  (2)

On the other hand, in this method, the read speed of information islimited by the frequency of the alternating current electric field, andtherefore, information cannot be read at a bit rate that is equal to ormore than the frequency of the alternating current electric field. Forexample, in order to achieve a read speed of 1 Gbit/second or more, itis necessary to apply an alternating current electric field of 1 GHz ormore. In this case, the dielectric constant of the dielectric depends onthe frequency of the alternating current electric field, and the higherthe frequency is, the greater the loss becomes. Therefore, theferroelectric that can be used in the ferroelectric rotating medium thatcan realize a high-speed operation is limited.

In the present embodiment, without using the alternating currentelectric field to read the information from the ferroelectric recordingmedium, charge of the ferroelectric layer 131 is detected by a weaktunnel current flowing between the conductive probe 26 and the electrodelayer 12.

Specifically, because the conductive probe 26 is very close to thesurface of the ferroelectric layer 131, the capacitance C, due to theferroelectric layer 131, between the conductive probe and the electrodelayer 12 is expressed by the following expression (3).C=ε·ε ₀ A/d,where A denotes a relative area of the conductive probe, d denotes athickness of the ferroelectric layer, ε denotes a relative dielectricconstant of the ferroelectric layer, and ε₀ denotes a dielectricconstant in vacuum  (3)

Then, where charge accumulated in the ferroelectric layer 131 is denotedas Q, a voltage V occurring at the conductive probe 26 is expressed bythe following expression (4). By detecting the voltage V, informationwritten to the ferroelectric layer 131 can be read.V=Q/C  (4)

In this case, because the ferroelectric is an insulator and has a largeband gap, and accordingly a tunnel current is less likely to flow, it isdifficult to detect the voltage V of the above-described expression (4).However, when the ferroelectric is made into a thin film to reduce thetunnel barrier, and a bonding structure with the electrode layer that isa conductor is formed, a weak tunnel current flows from the electronstate at the bonding portion. Further, when a paraelectric layer isadded to the bonding portion to make a bonding structure in which theferroelectric, the paraelectric, and the conductor are bonded in thisorder, band bending occurs in a direction for reducing the tunnelbarrier due to charge at the interface portion between the ferroelectricand the paraelectric, and accordingly, the tunnel current is more likelyto flow.

Furthermore, the tunnel barrier of the ferroelectric layer 131 changesdue to the polarization direction, and therefore, the polarizationdirection of the ferroelectric layer 131 can be found by measuring thetunnel current between the ferroelectric layer 131 and the conductiveprobe 26. For example, in any given ferroelectric, when the surfacelayer side is positively charged, the tunnel barrier increases, and thetunnel current from the side of the ferroelectric layer 131 towards theconductive probe 26 decreases, and conversely, when the surface layerside is negatively charged, the tunnel current from the side of theferroelectric layer 131 towards the conductive probe 26 increases.

When the recording-and-reproduction signal processing unit 50 performsreading by detecting the charge of the ferroelectric layer 131, therecording-and-reproduction signal processing unit 50 may apply a biasvoltage to the conductive probe 26.

The bias voltage applied to the conductive probe 26 is used tofacilitate detection of the tunnel current between the ferroelectriclayer 131 and the conductive probe 26, and can reduce variation of theamount of charge accumulated in the ferroelectric layer 131 wheninformation is read.

A positive or negative voltage, or both, can be used for the biasvoltage. In a case where the bias applied to the conductive probe 26 isa constant voltage of a positive or negative voltage, the polarizationdirection of the ferroelectric layer 131 is detected by the magnitude ofthe tunnel current that occurs due to application of the bias.

For example, in a ferroelectric of which the tunnel barrier increaseswhen the surface layer side is positively charged and the tunnel barrierdecreases when the surface layer side is negatively charged, in a casewhere a bias voltage is applied so that a tunnel current flows from theside of the ferroelectric layer 131 (the side of the electrode layer 12)towards the conductive probe 26, the magnitude of the tunnel barrier andthe magnitude of the tunnel current are in opposite relationship, basedon which the direction of the charge of the ferroelectric layer 131 canbe detected.

Also, in a case where the bias voltage applied to the conductive probe26 is both of the positive and negative voltages, the polarizationdirection of the ferroelectric layer 131 can be detected by comparingthe tunnel current that occurs when the positive bias voltage is appliedand the tunnel current that occurs when the negative bias voltage isapplied. The bias voltage in this case is preferably a sine wave or asquare wave of which the frequency is equal to or more than N Hz (N is anumber of 1 or more) where the read speed of information from theferroelectric layer 131 is denoted as N bits/second (N is a number of 1or more). In this manner, the polarization direction of theferroelectric layer 131 can be detected with a more high degree ofaccuracy. The reason for this is as follows. The tunnel current changesaccording to the distance between the ferroelectric layer 131 and theconductive probe 26, and therefore, in order to detect the polarizationdirection of the ferroelectric layer 131, it would be necessary todistinguish the above change and a change in the tunnel current due tothe polarization direction of the ferroelectric layer 131. Conversely,in a case where positive and negative bias voltages are used, thepolarization direction of the ferroelectric layer 131 is determinedthrough relative comparison of the tunnel current in the positive biasstate and the negative bias state, and accordingly, variation of thetunnel current due to the distance between the ferroelectric layer 131and the conductive probe 26 is cancelled, and therefore, thedetermination is less likely to be affected by the influence thereof.

Furthermore, in a case where, when charge of the ferroelectric layer 131is detected to read information, the amount of charge accumulated in theferroelectric layer 131 decreases in the ferroelectric storage apparatus100, and the tunnel current obtained therefrom decreases, therecording-and-reproduction signal processing unit 50 may rewrite (i.e.,perform refresh of) information that is the same as the informationwritten in the ferroelectric recording medium 10 to the position of theferroelectric recording medium 10 where the same information is read, inorder to compensate the charge that has decreased due to reading ofinformation from the ferroelectric recording medium 10. Note thatdecrease in the amount of charge accumulated in the ferroelectric layer131 also occurs when charge is captured by a defect included in theferroelectric layer 131.

The recording-and-reproduction signal processing unit 50 may performrewrite on every reading of information from the ferroelectric recordingmedium 10, or may perform rewrite after a predetermined number ofreadings is performed.

The reading of information from the ferroelectric layer 131 explainedabove is based on a non-destructive method. Alternatively, for readingof information from the ferroelectric layer 131, a destructive methodmay be employed.

In a case where the non-destructive method is used for reading ofinformation, the electric field occurring due to the bias applied to theconductive probe 26 does not exceed the coercive electric field of theferroelectric constituting the ferroelectric layer 131, and therefore,during reading of information, the polarization direction of theferroelectric layer 131 does not change.

In contrast, in a case where the destructive method is used, informationis read by applying a bias voltage that exceeds the coercive electricfield of the ferroelectric to the conductive probe 26 and detecting thetunnel current that occurs when the polarization direction of theferroelectric layer 131 changes in response to the application of thebias voltage. For example, in a case where the side of the ferroelectriclayer 131 on the side of the conductive probe 26 has a positive chargeand a negative bias voltage is applied to the conductive probe 26,charge of the ferroelectric layer 131 is not inverted, and accordingly,the tunnel current flowing from the ferroelectric layer 131 to theconductive probe 26 is small. Conversely, in a case where theferroelectric layer 131 has a negative charge, the charge of theferroelectric layer 131 is positively inverted due to the negative biasapplied to the conductive probe 26, and accordingly, the tunnel currentincreases. Due to this variation in the tunnel current, informationrecorded in the ferroelectric layer 131 can be read. Note that when thedestructive method is used for reading of information, it is necessaryto rewrite read information to the ferroelectric layer 131.

For reading of information from the ferroelectric recording medium 10,an atomic force between the conductive probe 26 and the ferroelectriclayer 131 may be used. A method using an atomic force between theconductive probe 26 and the ferroelectric layer 131 for reading ofinformation from the ferroelectric recording medium 10 is explained. Anatomic force between the conductive probe 26 and the ferroelectric layer131 is affected by charge of the ferroelectric layer 131, and therefore,information recorded in the ferroelectric recording medium 10 is read bymeasuring the atomic force therebetween. In this case, unlike the casewhere the tunnel current is used for reading information, the amount ofcharge accumulated in the ferroelectric layer 131 is less likely toflow, and therefore, it is not necessary to perform refresh. Also, it isnot necessary to consider the interface electron state between theelectrode layer 12 and the band gap of the ferroelectric used for theferroelectric layer 131.

In this case, an atomic force microscope (AFM) is known as an apparatusfor detecting the atomic force exerted between a probe and a surface ofa sample and mapping the atomic force. In the AFM, an optical levermethod is used, and while the sample is moved in the X-Y axis direction,laser light is emitted to a cantilever provided with the probe, and theinteratomic force is detected from the transition of the reflectedlight. Because this method detects physical variation of the cantilever,it is difficult to use this method for reading information in the GHzband. Furthermore, because laser light is used for detection, it mayinduce an internal photoelectric effect and temperature rise of aferroelectric used for a ferroelectric recording medium.

Therefore, in the present embodiment, as illustrated in FIG. 28 , thethird piezoelectric element 27C may be provided between the probe slider23 and the conductive probe 26. Accordingly, the third piezoelectricelement 27C detects an atomic force between the conductive probe 26 andthe ferroelectric layer 131, and this detected atomic force can beconverted into an electric signal. In this case, the optical lever isnot used for detection of the atomic force, and therefore, informationcan be read in the GHz band.

The third piezoelectric element 27C may be constituted by, for example,quartz, lithium niobate (LiNbO₃), barium titanate (BaTiO₃), titanatelead zirconate (PZT), zinc oxide (ZnO), aluminum nitride (AlN), lithiumtantalate (LiTaO₃), lead titanate (PT), and the like. The thirdpiezoelectric element 27C may be constituted by the same material as thefirst piezoelectric element 27A and the second piezoelectric element27B.

In order to increase the detection performance of the atomic force, thethird piezoelectric element 27C is preferably brought closer to theconductive probe 26. For this reason, as illustrated in FIG. 28 , thethird piezoelectric element 27C is preferably provided between theconductive probe 26 and the fourth piezoelectric element 27D. Similar tothe first piezoelectric element 27A, the fourth piezoelectric element27D is used to drive the conductive probe 26 in the +Z axis direction orthe −Z axis direction and bring the conductive probe 26 into proximitywith the surface of the ferroelectric recording medium 10. Note that thefourth piezoelectric element 27D may be constituted by the same materialas the third piezoelectric element 27C.

[Housing]

As illustrated in FIG. 8 , the housing 60 is formed in a substantiallyrectangular shape. The ferroelectric recording medium 10, the conductiveprobe 26, the probe slider 23, the ferroelectric recording mediumdriving unit 30, and the recording-and-reproduction signal processingunit 50 are provided in the housing 60.

In the ferroelectric storage apparatus 100, the housing 60 is preferablyfilled with at least one of argon gas, nitrogen gas, and helium gas.Charge occurs in friction between objects caused by movement of theferroelectric recording medium 10, the ferroelectric recording mediumdriving unit 30, and the conductive probe 26 and in friction betweenthese objects and air, i.e., what is termed as triboelectric chargingoccurs in the housing 60, and this charge may couple with the chargerecorded in the ferroelectric recording medium 10, so that writteninformation may be lost, and also reading and writing of information bythe conductive probe 26 may be adversely affected. In the presentembodiment, the housing 60 is filled with these gases, that canalleviate triboelectric charging.

The triboelectric charging in the housing 60 can be evaluated bymeasuring the number of charges of triboelectric charging according to aconventional measurement method.

In this manner, the ferroelectric storage apparatus 100 includes theferroelectric recording medium 10, the conductive probe 26, the probeslider 23, the ferroelectric recording medium driving unit 30, and therecording-and-reproduction signal processing unit 50. Therecording-and-reproduction signal processing unit 50 can recordmulti-value information to the ferroelectric recording medium 10, andreproduce recorded multi-value information. Multi-value information canbe recorded to the recording area of the smallest size on theferroelectric layer 131 of the ferroelectric recording layer 13 providedin the ferroelectric recording medium 10.

In the ferroelectric storage apparatus 100, therecording-and-reproduction signal processing unit 50 can recordmulti-value information to the ferroelectric recording medium 10, andaccordingly, the recording density of the ferroelectric recording medium10 can be increased. In the ferroelectric storage apparatus 100, therecording density of the ferroelectric recording medium 10 is increased,so that the size of the ferroelectric storage apparatus 100 can bereduced with respect to the unit storage capacity, and the speed (readand write speed) required for recording information to the ferroelectriclayer 131 and reproducing recorded information can be increased to, forexample, 10 Gbps or more.

Also, in the ferroelectric storage apparatus 100, the recording densityof the ferroelectric recording medium 10 is increased, so that theincrease in the power consumption with respect to the unit storagecapacity required for recording information, to and reproducinginformation from the ferroelectric recording medium 10 can bealleviated.

Therefore, the ferroelectric storage apparatus 100 can achieve animprovement of the recording density, a reduction in the size withrespect to the unit storage capacity, and an increase in the read andwrite speed, and can alleviate an increase in the energy consumption.

In this case, in a conventional magnetic recording medium such as a HDD,while a magnetic recording medium is rotated at 5000 rpm to 10000 rpm(83 to 167 rotations per second), the read-and-write header is moved inthe track direction (the radius direction) to record (write) andreproduce (read) information. The size of one bit of information isapproximately 5 nm in the sector direction (circumferential direction)and 50 nm in the track direction, and this single bit includesapproximately 10 magnetic particles. The read-and-write speed isapproximately 1 Gbps on average. In the ferroelectric recording medium10, information is recorded by polarization inversion due to the latticestrain of the ferroelectric crystal included in the ferroelectric layer131. Therefore, as compared to a magnetic recording medium that recordsinformation by magnetizing the magnetic layer in units of magneticparticles, the recording density and the read-and-write speed can besignificantly increased. Furthermore, in the ferroelectric storageapparatus 100, the ferroelectric recording medium 10 has theferroelectric layer 131 that has the recording area in which multi-valuerecording is performed, and therefore, the recording density can beimproved, the size and the processing speed can be increased, and anincrease in the energy consumption can be alleviated.

The ferroelectric storage apparatus 100 has a higher recording densityand a higher read-and-write speed, and is small in size, so that theferroelectric storage apparatus 100 can achieve, for example, a storagefor wireless and mobile communication of 10 Gbps or more. Furthermore,the ferroelectric storage apparatus 100 alleviates an increase in thepower consumption and achieves energy saving, so that the consumption ofresources can be reduced, and accordingly, the environmental load can bereduced.

In the ferroelectric storage apparatus 100, therecording-and-reproduction signal processing unit 50 can read, from theferroelectric layer 131, position information (servo information) fordetecting relative position, in the track direction of the ferroelectricrecording medium 10, between the conductive probe 26 and theferroelectric recording medium 10. Accordingly, the ferroelectricstorage apparatus 100 can accurately detect the position of theconductive probe 26 according to the servo information duringreproduction of data recorded in data area of the ferroelectric layer131 provided in the ferroelectric recording medium 10, and therefore,the ferroelectric storage apparatus 100 can record information to andreproduce information from the ferroelectric layer 131 with a highdegree of accuracy. Therefore, the ferroelectric storage apparatus 100increases the processing speed during recording of information andreproduction of recorded information.

In the ferroelectric storage apparatus 100, the servo information arearecorded with servo information and the data area in which informationis written and read can be arranged alternately in the circumferentialdirection of the track on the ferroelectric recording medium 10.Accordingly, in the ferroelectric storage apparatus 100, the controlunit, not illustrated, can accurately detect the position of theconductive probe 26 according to servo information for each of the dataareas during reproduction of data recorded in the data area. Therefore,the ferroelectric storage apparatus 100 can increase the accuracy ofwriting of information to and reading of information from theferroelectric layer 131, so that the processing speed during recordingof information and reproduction of recorded information can be furtherincreased.

In the ferroelectric storage apparatus 100, the reference signalinformation 131B-4 can be included in the servo information area 131B ofthe ferroelectric layer 131 of the ferroelectric recording medium 10.Accordingly, the ferroelectric storage apparatus 100 uses the signallevels of multi-value recording that are ascertained by reading thereference signal information 131B-4 with the conductive probe 26, sothat the multi-value information recorded in the data area 131A can bereproduced. Therefore, the ferroelectric storage apparatus 100 canfurther reduce the energy consumption with respect to the unit storagecapacity.

According to a method for writing information to and reading informationfrom the ferroelectric storage apparatus 100 having the above-describedconfiguration, multi-value information can be written to and recordedmulti-value information can be read from the ferroelectric recordingmedium 10 with the simplest single operation with respect to therecording area of the smallest size. When the above-described method forwriting and reading information is used, in therecording-and-reproduction signal processing unit 50, the recordingdensity stored in the ferroelectric layer 131 can be improved, the sizeof the ferroelectric storage apparatus 100 with respect to the unitstorage capacity can be reduced, the read-and-write speed can beincreased, and an increase in the energy consumption can be alleviated.

In the ferroelectric storage apparatus 100, the first piezoelectricelement 27A and the second piezoelectric element 27B can be provided onthe probe slider 23. The ferroelectric storage apparatus 100 uses thepair of electrodes 28A-1 and 28A-2 to cause the first piezoelectricelement 27A to expand and shrink in the height direction of the firstpiezoelectric element 27A, and uses the electrodes 28B-1 and 28B-2 tocause the second piezoelectric element 27B to expand and shrink in thetrack direction of the ferroelectric recording medium 10. Accordingly,the ferroelectric storage apparatus 100 can control the distance betweenthe ferroelectric recording medium 10 and the conductive probe 26 in anorder of nanometers, and move the conductive probe 26 in the trackdirection of the ferroelectric recording medium 10 in an order ofnanometers. Also, a movement time within a same track and a movementtime (seek time) to another track can be reduced to several microsecondsor less. Therefore, the ferroelectric storage apparatus 100 can performpositioning of the conductive probe 26 in the data surface direction andthe track direction of the ferroelectric recording medium 10 with a highdegree of accuracy, i.e., an accuracy of 1 nm or less, and can perform acorrecting operation within a same track and a moving operation toanother track in several microseconds or less. Therefore, theferroelectric storage apparatus 100 can improve the recording capacityand increase the speed required for recording and reproduction ofinformation.

In the ferroelectric storage apparatus 100, the second piezoelectricelement 27B is provided between the probe slider 23 and the conductiveprobe 26, and the conductive probe 26 can be provided with a shift inposition in the track direction of the ferroelectric recording medium 10with reference to the center of the attachment surface of the secondpiezoelectric element 27B. Accordingly, the conductive probe 26 can movein the track direction according to expansion and shrinking of thesecond piezoelectric element 27B. Therefore, the ferroelectric storageapparatus 100 can increase the accuracy of positioning of the conductiveprobe 26 in the track direction of the ferroelectric recording medium10, and can reliably perform a correcting operation within a same trackand a moving operation to another track in several microseconds or less.Therefore, the ferroelectric storage apparatus 100 can further improvethe recording capacity, and can further increase the speed required forrecording and reproduction of information.

In a movement of the conductive probe 26 in the track direction of theferroelectric recording medium 10, the ferroelectric storage apparatus100 uses the probe driving unit 40 for a coarse operation of which themovement distance is 10 nm or more, and uses the second piezoelectricelement 27B for a fine movement of which the movement distance is lessthan 10 nm. Accordingly, the ferroelectric storage apparatus 100 canappropriately move the conductive probe 26 according to the movementdistance of the conductive probe 26 in the track direction of theferroelectric recording medium 10. Therefore, the ferroelectric storageapparatus 100 can further improve the recording density, and can furtherincrease the speed required for recording and reproduction ofinformation.

In the ferroelectric storage apparatus 100, in order to detect thecharge of the ferroelectric layer 131, the recording-and-reproductionsignal processing unit 50 applies positive and negative bias voltages tothe conductive probe 26, and detects charge of the ferroelectric layer131 by measuring a weak tunnel current flowing between the conductiveprobe 26 and the electrode layer 12. Accordingly, in the ferroelectricstorage apparatus 100, the recording-and-reproduction signal processingunit 50 can read information without using an alternating currentelectric field, and accordingly, the read speed is not limited by thefrequency of the alternating current electric field. Therefore,information stored in the ferroelectric recording medium 10 can be readat a high speed.

In the ferroelectric storage apparatus 100, in order to detect charge ofthe ferroelectric layer 131, the recording-and-reproduction signalprocessing unit 50 applies a positive or negative bias voltage to theconductive probe 26. By applying a voltage to the conductive probe 26,the ferroelectric storage apparatus 100 can detect charge accumulated inthe ferroelectric layer 131, and therefore, the ferroelectric storageapparatus 100 can read information stored in the ferroelectric layer 131at a high speed.

In the ferroelectric storage apparatus 100, in order to detect charge ofthe ferroelectric layer 131, the recording-and-reproduction signalprocessing unit 50 applies positive and negative bias voltages to theconductive probe. Accordingly, the ferroelectric storage apparatus 100can detect the polarization direction of the ferroelectric layer 131 bycomparing a tunnel current when a positive bias voltage is applied and atunnel current when a negative bias voltage is applied. Therefore, byapplying a voltage to the conductive probe 26, the ferroelectric storageapparatus 100 can facilitate detection of charge accumulated in theferroelectric layer 131, and therefore, the ferroelectric storageapparatus 100 can more reliably read information stored in theferroelectric layer 131.

The ferroelectric storage apparatus 100 can use a sine wave or a squarewave as positive and negative bias voltages. Accordingly, theferroelectric storage apparatus 100 can furthermore facilitate detectionof the polarization direction of the ferroelectric layer 131.Accordingly, by applying a voltage to the conductive probe 26, theferroelectric storage apparatus 100 can furthermore facilitate detectionof charge accumulated in the ferroelectric layer 131. Therefore,information stored in the ferroelectric layer 131 can be read morereliably.

According to a method for reading information with the ferroelectricstorage apparatus 100 having the above-described configuration, chargeof the ferroelectric layer 131 is detected by comparing a tunnel currentflowing between the conductive probe 26 and the ferroelectric layer 131when a positive bias voltage is applied to the conductive probe 26 and atunnel current flowing between the conductive probe 26 and theferroelectric layer 131 when a negative bias voltage is applied to theconductive probe 26. When the above-described method for readinginformation is used, the recording-and-reproduction signal processingunit 50 can more easily detect charge accumulated in the ferroelectriclayer 131, and can more reliably read information stored in theferroelectric layer 131.

According to a method for reading information with the ferroelectricstorage apparatus 100 having the above-described configuration, in acase where the read speed of information is N bits/second (N is a numberof 1 or more), the frequency of the applied bias voltage can be set to NHz (N is a number of 1 or more) or more. Accordingly, when theabove-described method for reading information is used, therecording-and-reproduction signal processing unit 50 can furthermorefacilitate detection of charge accumulated in the ferroelectric layer131, and therefore, the recording-and-reproduction signal processingunit 50 can more reliably read information stored in the ferroelectriclayer 131.

According to a method for writing information to and reading informationfrom the ferroelectric storage apparatus 100 having the above-describedconfiguration, information that is the same as the information writtenin the ferroelectric recording medium 10 is rewritten to the position ofthe ferroelectric recording medium 10 where the same information isread. When the above-described method for writing and readinginformation is used, charge that has decreased due to reading ofinformation from the ferroelectric recording medium 10 can becompensated for, and therefore, when the recording-and-reproductionsignal processing unit 50 measures a weak tunnel current flowing betweenthe conductive probe 26 and the electrode layer 12, therecording-and-reproduction signal processing unit 50 can stably detectcharge of the ferroelectric layer 131.

According to a method for writing information to and reading informationfrom the ferroelectric storage apparatus 100 having the above-describedconfiguration, rewriting can be performed on every reading ofinformation from the ferroelectric recording medium 10, or can beperformed after a predetermined number of reading is performed.Accordingly, charge that has decreased due to reading of informationfrom the ferroelectric recording medium 10 can be compensated for on anygiven reading, and therefore, rewriting can be performed appropriatelywhen necessary according to the amount of decrease in the charge of theferroelectric layer 131.

The ferroelectric storage apparatus 100 can make the shape of theconductive probe 26 into a cone shape. Accordingly, the conductive probe26 has a sharp tip, so that a higher degree of sharpening can beachieved, and therefore, the electric field strength increases, and thevoltage applied to the needle-shaped electrode 262 can be reduced.Therefore, the ferroelectric storage apparatus 100 can advantageouslyperform recording information to the ferroelectric recording medium 10.

The ferroelectric storage apparatus 100 can make the shape of theconductive probe 26A into a triangular pyramid or quadrangular pyramidshape. Accordingly, the ferroelectric storage apparatus 100 can have theconductive probe 26 having a sharp tip.

The ferroelectric storage apparatus 100 can make the shape of theconductive probe 26A into a rotational symmetrical shape about the axisthat passes through the tip thereof. Accordingly, the conductive probe26A can uniformize the electric field distribution that occurs at thetip of the conductive probe 26A during writing, and can stabilize thetunnel current that flows between the conductive probe 26A and theferroelectric layer 131 during reading. Therefore, the ferroelectricstorage apparatus 100 can stably write information to the ferroelectricrecording medium 10 and can stably read information from theferroelectric recording medium 10.

The manufacturing method of the conductive probe 26A may include a stepof forming a mask in a triangular or quadrangular shape on the surfaceof the conductive material 260 and a step of obtaining the needle-shapedelectrode 262 formed in a triangular pyramid or quadrangular pyramidshape by etching the conductive material 260. Accordingly, theconductive probe 26A including the needle-shaped electrode 262 formed ina triangular pyramid or quadrangular pyramid shape on the base body 261can be manufactured with a high degree of reproducibility.

Also, a manufacturing method of the conductive probe 26A may include astep of forming the mask 72A having the through hole 72 a in atriangular or quadrangular shape in the surface of the conductivematerial 260 and a step of obtaining the needle-shaped electrode 262Aformed in a triangular pyramid or quadrangular pyramid by depositing aconductive material on the surface of the conductive material 260 in thethrough hole 72 a. Accordingly, the conductive probe 26A having theneedle-shaped electrode 262A formed in a triangular pyramid orquadrangular pyramid shape on the base body 261 can be manufactured witha high degree of reproducibility.

In the ferroelectric storage apparatus 100, the conductive probe 26B mayinclude the base body 261 constituted by the conductive material 260,the recessed portion 212, and the needle-shaped electrode 213 made intoa sharpened shape, wherein a portion of the needle-shaped electrode 213may protrude from the surface of the conductive material 260.Accordingly, the ferroelectric storage apparatus 100 can prevent damageto the needle-shaped electrode 262 made into a sharpened shape.Furthermore, the ferroelectric storage apparatus 100 can alleviatevibration and deformation of the needle-shaped electrode 262 caused byair flow that occurs due to rotation of the ferroelectric recordingmedium 10. Furthermore, the ferroelectric storage apparatus 100 shieldsthe needle-shaped electrode 262 and can alleviate the influence ofcharge around the needle-shaped electrode 262 and leakage of charge fromthe needle-shaped electrode 262.

In the ferroelectric storage apparatus 100, the conductive probe 26C mayinclude the base body 261 constituted by the conductive material 260,the insulating layer 263 provided on the base body 261 and having thethrough hole 263 a, and the needle-shaped electrode 262 formed in a coneshape on the base body 261 in the through hole 263 a, wherein a portionof the needle-shaped electrode may protrude from the surface of theinsulating layer 263. Even in this case, the ferroelectric storageapparatus 100 can alleviate damage to the needle-shaped electrode 262.Also, the ferroelectric storage apparatus 100 can alleviate vibrationand deformation of the needle-shaped electrode 262 caused by air flowthat occurs due to rotation of the ferroelectric recording medium 10.Furthermore, the ferroelectric storage apparatus 100 shields theneedle-shaped electrode 262 and can alleviate the influence of chargearound the needle-shaped electrode 262 and leakage of charge from theneedle-shaped electrode 262.

A manufacturing method of the conductive probe 26B may include a step ofapplying a photoresist to the surface of the conductive material 260, astep of forming a very small through hole (hole) in the photoresist, astep of forming a recess (recessed portion) in a concave shape byetching the surface of the conductive material 260 in the hole via thehole, a step of depositing metal on the photoresist having the hole, anda step of obtaining the needle-shaped electrode 262 formed in a coneshape by removing a photoresist, wherein a portion of the needle-shapedelectrode 262 may protrude from the conductive material 260.Accordingly, the conductive probe 26B can be manufactured with a highdegree of reproducibility.

The manufacturing method of the conductive probe 26C may include a stepof forming the insulating layer 263, by oxidizing the conductivematerial 260, on the conductive material 260, a step of forming theisolation layer 29 on the insulating layer 263, a step of applying thephotoresist 82 on the surface of the isolation layer 29, a step offorming the through hole 82 a in the photoresist 82, a step of etchingthe through hole 82 a to the surface of the conductive material 260, astep of obtaining the needle-shaped electrode 262 by depositing metal onthe surface of the conductive material 260 in the through hole 82 a, anda step of removing the photoresist 82, wherein a portion of theneedle-shaped electrode 262 may protrude from the insulating layer 263.Even in this case, the conductive probe 26C can be manufactured with ahigh degree of reproducibility.

Also, the manufacturing method of the conductive probe 26C includes astep of forming a mask in a triangular or quadrangular shape on thesurface of the conductive material 260 and a step of obtaining theneedle-shaped electrode 262 formed in a triangular pyramid orquadrangular pyramid shape by etching the conductive material 260.Accordingly, the conductive probe 26C including the needle-shapedelectrode 262 formed in a triangular pyramid or quadrangular pyramidshape on the base body 261 can be manufactured with a high degree ofreproducibility.

Also, the manufacturing method of the conductive probe 26C may include astep of forming a mask having the through hole 72 a in a triangular orquadrangular shape on the surface of the conductive material 260 and astep of forming the needle-shaped electrode 262 formed in a triangularpyramid or quadrangular pyramid shape by depositing a conductivematerial on the surface of the conductive material 260 in the throughhole 72 a. Accordingly, the conductive probe 26C including theneedle-shaped electrode 262 formed in a triangular pyramid orquadrangular pyramid shape formed on the base body 261 can bemanufactured with a high degree of reproducibility.

Specifically, in the manufacturing method of the conductive probe 26,the hole of the photoresist may be formed in a circular shape, atriangular shape, and a quadrangular shape, when the conductive probe 26is manufactured using a photoresist. Accordingly, by removing thephotoresist after metal is deposited on the photoresist, the conductiveprobe 26 including the needle-shaped electrode 262 formed in a coneshape, triangular pyramid, or quadrangular pyramid on the base body 261can be manufactured with a high degree of reproducibility.

In the ferroelectric storage apparatus 100, therecording-and-reproduction signal processing unit 50 can detect a signalgenerated by an atomic force between the conductive probe 26 andferroelectric recording medium 10. Accordingly, in the ferroelectricstorage apparatus 100, the recording-and-reproduction signal processingunit 50 can read information without using an alternating currentelectric field, and accordingly, the read speed is not limited by thefrequency of the alternating current electric field. Therefore,information stored in the ferroelectric recording medium 10 can be readat a high speed. Furthermore, the amount of charge accumulated in theferroelectric layer 131 does not decrease, and accordingly, re-readingcan be omitted.

The ferroelectric storage apparatus 100 can detect an atomic force withthe piezoelectric element 27 provided between the probe slider 23 andthe conductive probe 26. Accordingly, the ferroelectric storageapparatus 100 can increase the detection performance of an atomic force,and therefore, information stored in the ferroelectric recording medium10 can be read at a higher speed.

The ferroelectric storage apparatus 100 includes the piezoelectricelement 27 or an electrostrictive element and the control unit, notillustrated. In the ferroelectric storage apparatus 100, the controlunit, not illustrated, causes the piezoelectric element 27 or theelectrostrictive element to expand and shrink by controlling a voltageapplied to the piezoelectric element 27 or the electrostrictive elementon the basis of a read signal from the conductive probe 26. Accordingly,in the ferroelectric storage apparatus 100, the distance between theferroelectric recording medium 10 and the conductive probe 26 can beadjusted, and therefore, the distance between the conductive probe 26and the ferroelectric recording medium 10 can be controlled at a highdegree of accuracy.

In this case, FIG. 29 illustrates a cross-sectional view illustrating anexample of a configuration of a conventional magnetic header slider 110.As illustrated in FIG. 29 , the magnetic header slider 110 used for anHDD includes a heating body 111 provided in the magnetic header slider110 and a magnetic header 112 provided below the heating body 111 toface the magnetic recording medium 120. The magnetic header slider 110uses a technique for energizing the heating body 111 to generate heatand thermally expand the magnetic header slider 110, thereby adjustingthe distance between magnetic header 112 and magnetic recording medium120, i.e., DFH (for example, see Japanese Patent Laid-Open No.2003-168274). The heating body 111 is referred to as a DFH heater, andthe power applied to the heating body 111 is referred to as the DFHpower. According to this technique for adjusting the DFH, while theamount of floating of the magnetic header slider 110 on the surface ofthe magnetic recording medium 120 is maintained in an order ofnanometers, the distance between the surface of the magnetic recordingmedium 120 and the magnetic header 112 is reduced to an order ofsub-nanometers. However, in a case where the heating body 111 is used,the heating range widely covers the magnetic header 112 and the magneticheader slider 110 on which the heating body 111 is mounted. Accordingly,it takes a long time to heat the magnetic header 112 with the heatingbody 111, and the response and the accuracy of the DFH control are nothigh. Furthermore, the leakage electric field in the ferroelectricrecording medium 10 is smaller than the leakage magnetic field used forreading magnetic information in the magnetic recording medium 120, sothat the ferroelectric storage apparatus 100 detecting the leakagemagnetic field is required to have an adjusting technique of the DFHwith a relatively higher degree of accuracy.

Furthermore, the conventional DFH technique as illustrated in FIG. 29uses the heating body 111, and therefore, when this technique is used,the dielectric constant of the ferroelectric layer 131 may thermallyvary. In contrast, the ferroelectric storage apparatus 100 does not havethe heating body 111 as in the conventional DFH technique, andtherefore, thermal variation of the dielectric constant of theferroelectric layer 131 can be prevented.

In the ferroelectric storage apparatus 100, the control unit, notillustrated, can cause the piezoelectric element 27 or theelectrostrictive element to expand and shrink by controlling the voltageapplied to the piezoelectric element 27 or the electrostrictive elementon the basis of a read signal from the conductive probe 26. Accordingly,the ferroelectric storage apparatus 100 can more readily control thedistance between the ferroelectric recording medium 10 and theconductive probe 26 with a high degree of accuracy at a high speed.

In the ferroelectric storage apparatus 100, the ferroelectric recordingmedium driving unit 30 includes the housing 31, the bearing sleeve 32,the spindle shaft 33, the housing bottom portion 34, the permanentmagnet 35, the stator 36, and the lubricant oil O. The curvature radiusof at least one of the shaft end portion 331 of the spindle shaft 33 andthe housing bottom portion 34 has a spherical surface in a convex shapeor concave shape of 2 mm or more, and the lubricant oil O includesconductive powder of the inorganic matter. Accordingly, theferroelectric recording medium driving unit 30 is a fluid dynamicbearing that is a type of plain bearing, and the spindle shaft 231 canbe rotated stably in a non-contact manner by a dynamic pressure thatoccurs during rotation of the spindle shaft 231 with the lubricant oil Othat fills the space between the spindle shaft 231 and the housing 31,and therefore, the ferroelectric recording medium 10 can be rotationallydriven with less vibration and less axial runout. Furthermore, theconductive powder of the inorganic matter has a high heat resistance andvolatility resistance, and does not increase the viscosity of thelubricant oil O even when the conductive powder is contained in thelubricant oil O. Therefore, the ferroelectric storage apparatus 100 canstabilize the torque of the ferroelectric recording medium driving unit30 even when it is used for a long period of time, and a reduction in aconduction between the spindle shaft 231 and the housing 31 can bealleviated. Therefore, the ferroelectric storage apparatus 100 canstabilize reading and writing of information with respect to theferroelectric recording medium 10.

Conventionally, the spindle shaft 33 does not come into contact with thehousing 31 due to the intervention of the fluid layer of lubricant fluidthat is non-conductive, and therefore, the spindle shaft 33 is anelectrically floating state from the housing 31 of the ferroelectricrecording medium driving unit. In this case, the spindle shaft 33 isconnected to the ferroelectric layer 131 via the ferroelectric recordingmedium, and constitutes a portion of the circuit that writes informationto the ferroelectric layer 131. Therefore, the state in which thespindle shaft 33 is electrically floating from the housing 31 causes aproblem in applying a voltage across the ferroelectric recording medium10 and the conductive probe 26 to write information.

Also, there is a method of using a lubricant fluid O containing aconductive material (for example, see Japanese Patent Laid-Open No.2001-208069), but an addition of the conductive material increases theviscosity of the lubricant oil O, which leads to an increase in thebearing torque. When used over a long period of time, the conductivematerial deteriorates and the conduction decreases, and therefore, evenif it is applied to the ferroelectric recording medium 10, the errorrate during writing of information to the ferroelectric recording medium10 is likely to increase.

Because the ferroelectric storage apparatus 100 includes theabove-described configuration, the torque is stabilized even when theferroelectric recording medium driving unit 30 is used over a longperiod of time, and a decrease of the conduction between the spindleshaft 231 and the housing 31 can be alleviated, and therefore, readingof information and writing of information with respect to theferroelectric recording medium 10 can be performed stably.

In the ferroelectric storage apparatus 100, the curvature radius of atleast one of the shaft end portion 331 of the spindle shaft 33 and thehousing bottom portion 34 provided in the ferroelectric recording mediumdriving unit 30 can be made into a spherical surface in a convex shapeor concave shape of 2 mm or more. Accordingly, in the ferroelectricstorage apparatus 100, the torque is stabilized even when theferroelectric recording medium driving unit 30 is used over a longperiod of time, and a decrease of the conduction between the spindleshaft 231 and the housing 31 can be alleviated.

The ferroelectric storage apparatus 100 can include a groove portion231B in a V shape on the outer circumference of the spindle shaft 33.Accordingly, when the spindle shaft 33 rotates, the lubricant oil O canbe readily collected at the vertex of the V shape of the groove portion231B, and accordingly, a flow is likely to occur in the lubricant oil O.Therefore, a pressure is likely to be generated by the lubricant oil O,so that the spindle shaft 33 is likely to be supported.

In the ferroelectric storage apparatus 100, the permanent magnet 35 andthe stator 36 can be provided to face each other in the cover 37. Thespindle shaft 33 can be rotated by the attraction between the permanentmagnet 35 and the electromagnet of the stator 36, and furthermore, inthe spindle shaft 33, a thrust toward the lower side can be generated,and accordingly, the spindle shaft 33 can be reliably supported by thehousing bottom portion 34.

In the ferroelectric storage apparatus 100, therecording-and-reproduction signal processing unit 50 can adjust avoltage waveform applied to the conductive probe 26 for writinginformation to the ferroelectric recording layer 13 to any one of atriangle wave, a sawtooth wave, and a trapezoidal wave. Accordingly, theferroelectric storage apparatus 100 can alleviate damage to theconductive probe 26.

The ferroelectric storage apparatus 100 includes the housing 60, and thehousing 60 may be filled with at least one of argon gas, nitrogen gas,and helium gas. The ferroelectric recording medium 10, the conductiveprobe 26, the probe slider 23, the ferroelectric recording mediumdriving unit 30, and the recording-and-reproduction signal processingunit 50 are accommodated in the housing 60, and the housing 60 is filledwith these gasses, so that the ferroelectric storage apparatus 100 canalleviate triboelectric charging that occurs in the housing 60.Therefore, the ferroelectric storage apparatus 100 can alleviate loss ofwrite information due to charge coupling with charge recorded in theferroelectric recording medium 10 and adverse effects exerted on writingby the conductive probe 26.

<Data Management System>

A data management system including the above-described ferroelectricstorage apparatus 100 as an external storage apparatus is explained.FIG. 30 is a drawing illustrating a configuration of a data managementsystem. As illustrated in FIG. 30 , the data management system 300 is adata management system for managing data on a high-speed communicationnetwork, and includes a data management unit 310 and at least oneexternal storage apparatus 320.

As illustrated in FIG. 30 , the data management unit 310 includes aninternal storage apparatus 311.

The data management unit 310 stores, to the external storage apparatus320, a large amount of data that flows at a high speed on a high-speedcommunication network as it is, and stores and saves, in the internalstorage apparatus 311, metadata used for reading of saved data.Accordingly, the data management unit 310 can save, to the externalstorage apparatus 320, a large amount of data flowing on a high-speedcommunication network at a high speed. The external storage apparatus320 can increase the expandability of the data management system becausethe external storage apparatus 320 can be more easily replaced than theinternal storage apparatus 311. In addition, the data management unit310 can readily read data saved in the external storage apparatus 320 byusing metadata saved in the internal storage apparatus 311.

The metadata is data saved in the internal storage apparatus 311 anddescribing supplementary information of data saved in the externalstorage apparatus 320. Specifically, the metadata includes a type, asize, an attribute, a format, a title, an author, a publishing company,a related keyword of data, a time when data occurs, a location, and thelike. Also, the metadata includes, for example, a position, a drivenumber, a track number, and a sector number in the external storageapparatus 320 that saves data.

As illustrated in FIG. 30 , the external storage apparatus 320 isconnected to the data management unit 310 via wires or wirelessly sothat data can be transmitted to and received from the data managementunit 310, and the external storage apparatus 320 stores data on thehigh-speed communication network transmitted from the data managementunit 310.

Multiple external storage apparatuses 320 can be provided. The multipleexternal storage apparatuses 320 are preferably arranged and provided inparallel. The multiple external storage apparatuses 320 can save a largeamount of data at a high speed, and reduce the load required for savingdata in each of the external storage apparatuses 320. Also, the multipleexternal storage apparatuses 320 are arranged and provided in parallel,so that the multiple external storage apparatuses 320 have a highexpandability, and therefore, the data capacity of data that can besaved can be increased.

FIG. 31 illustrates an example of a configuration of the externalstorage apparatus 320. As illustrated in FIG. 31 , the external storageapparatus 320 includes a ferroelectric recording medium 321, andpreferably further includes a storage apparatus 322, a read device 323,and a driving unit 324.

The ferroelectric recording medium 321 is substantially the same as theabove-described ferroelectric recording medium 10 (see FIG. 8 ), and thedetails thereof are omitted.

The storage apparatus 322 is a device for saving data to theferroelectric recording medium 321.

The read device 323 is a device for reading data from the ferroelectricrecording medium 321.

The storage apparatus 322 and the read device 323 are substantially thesame as the above-described conductive probe 26 (see FIG. 15 ) and thelike, and the details thereof are omitted.

The driving unit 324 includes a first driving unit 324A that drives thestorage apparatus 322 on a storage surface 321 a of the ferroelectricrecording medium 321 and a second driving unit 324B that drives the readdevice 323 on the storage surface 321 a. The first driving unit 324A andthe second driving unit 324B are substantially the same as theabove-described probe driving unit 40 (see FIG. 8 ), and the detailsthereof are omitted.

The storage apparatus 322 and the read device 323 may independentlydrive, with the driving unit 324, the same storage surface 321 a of theferroelectric recording medium 321.

The external storage apparatus 320 may save new data on the high-speedcommunication network upon deleting old data saved in the past withoutupdating the old data, or may save new data by overwriting old data.

Specifically, data saved in the external storage apparatus 320 ispreferably not data saved by overwriting old data saved in the past, andis rather preferably new data on the high-speed communication networkthat is saved by deleting or overwriting old data saved in the past.

The ferroelectric recording medium 321 is a rotating medium, andinformation is read from and information is written to the ferroelectricrecording medium 321 by moving a read-and-write device (the storageapparatus 322 and the read device 323) in the sector direction (thecircumferential direction) and the track direction (the radiusdirection) while the ferroelectric recording medium 321 is rotated at ahigh speed. In this method, it is most efficient to read informationfrom and write information to continuous sectors and continuous tracks.When information is read from or information is written to anothernon-continuous track or sector, it takes time to move the read-and-writedevice to another track, another sector, or the like. Therefore, inorder to update old data saved in the past, it takes time to move thewrite device to the track and sector where the data is stored.

The external storage apparatus 320 saves new data on the high-speedcommunication network upon deleting old data saved in the past withoutupdating the old data, or may save new data by overwriting old data, sothat the external storage apparatus 320 can write information to thecontinuous sector and the continuous track, and therefore, the externalstorage apparatus 320 can save data at a high speed.

FIGS. 32A and 32B illustrate an example of a connection relationship ofdata saved in the ferroelectric recording medium 321. As illustrated inFIG. 32A, the data saved in the external storage apparatus 320 is notsaved in a storage area having a hierarchical structure, but asillustrated in FIG. 32B, is saved in a centralized storage area thatdoes not have a hierarchical structure. Accordingly, the externalstorage apparatus 320 can reduce the time required to move the storageapparatus to another hierarchical position of the ferroelectricrecording medium 321 during writing of information, and therefore, theexternal storage apparatus 320 can store data to the ferroelectricrecording medium 321 at a high speed.

The hierarchical structure is a structure arranged in such a state thatmultiple pieces of data located at a lower hierarchical level of theferroelectric recording medium 321 are branched from a single piece ofdata that belongs to a given hierarchical level of the ferroelectricrecording medium 321.

In this manner, the data management system 300 includes the datamanagement unit 310 and at least one external storage apparatus 320. Thedata management unit 310 saves data on the high-speed communicationnetwork to the external storage apparatus 320, and saves metadata to theinternal storage apparatus 311. Multiple external storage apparatuses320 may be provided. The data management system 300 saves a large amountof data flowing at a high speed on the high-speed communication networkfrom the data management unit 310 to the external storage apparatus 320,and can easily read data saved in the external storage apparatus 320 byusing metadata saved in the internal storage apparatus 311. Also, theexternal storage apparatus 320 can be readily replaced and added, andmultiple external storage apparatuses 320 may be provided, so that thedata capacity of data that can be saved can be increased, and the burdenrequired to save data of the external storage apparatus 320 can bereduced. Accordingly, the data management system 300 can perform writingwith a high record density at a high speed, and can have a highexpandability. Therefore, the data management system 300 can efficientlysave data on the high-speed communication network, and can increase theconvenience.

The data management system 300 can be configured by connecting the datamanagement unit 310 to the high-speed communication network and canconnect the external storage apparatus 320 to the data management unit310. Accordingly, the data management system 300 can save data on thehigh-speed communication network via the data management unit 310 to theexternal storage apparatus 320, and can reliably save, to the internalstorage apparatus 311, metadata used for reading data stored in theexternal storage apparatus 320. Therefore, the data management system300 can efficiently save data on the high-speed communication network,and can read data saved in the external storage apparatus 320 at ahigher speed.

In the data management system 300, multiple external storage apparatuses320 can be arranged in parallel. Accordingly, the data management system300 can write data on the high-speed communication network to respectiveexternal storage apparatuses 320 in an evenly distributed manner.Therefore, the data management system 300 can more efficiently save alarge amount of data to the multiple external storage apparatuses 320 ata high speed, and can improve the expandability more greatly. Therefore,the data management system 300 can more greatly increase the conveniencewhile the data capacity of data that can be saved can be increased.

In the data management system 300, the external storage apparatus 320saves new data on the high-speed communication network upon deleting olddata saved in the past without updating the old data, or saves new databy overwriting old data. Accordingly, the data management system 300 canfacilitate writing of data to the ferroelectric recording medium 321 ofthe external storage apparatus 320, and facilitates reading of datasaved in the external storage apparatus 320. Therefore, the datamanagement system 300 can reliably save data to or read data from theexternal storage apparatus 320 at a high speed while maintaining thedata.

In the data management system 300, the external storage apparatus 320includes the storage apparatus 322, the read device 323, and the drivingunit 324. Accordingly, the data management system 300 can performwriting and reading of information in continuous sectors and continuoustracks on the same storage surface 321 a of the ferroelectric recordingmedium 321. Also, during writing of information in continuous sectorsand continuous tracks, sporadic read requests of information can behandled. Therefore, the data management system 300 can save data to theferroelectric recording medium 321 at a high speed, and can read saveddata.

EXAMPLES

Hereinafter, the embodiment is specifically explained by showingExamples and Comparative Examples, but the embodiment is not limited bythese Examples and Comparative Examples.

Example 1

[Production of Target for Forming Ferroelectric Layer]

(Production of Hf_(0.5)Zr_(0.5)O₂ Target)

A mixture obtained by mixing HfO₂ powder and ZrO₂ powder at a ratio of1:1 was made into a slurry using water as a solvent, and was thereafterspray-dried to manufacture a mixed powder. A molded product obtained bypressing this mixed powder was baked in an inert atmosphere tomanufacture a target. The density of the manufactured target was about96% of the theoretical value of Hf_(0.5)Zr_(0.5)O₂.

(Production of 4(Y₂O₃)-96(HfO₂) target)

The 4(Y₂O₃)-96(HfO₂) target was manufactured in the same manner as theabove-described Hf_(0.5)Zr_(0.5)O₂ target was manufactured, except thatthe Y₂O₃ powder and the hafnium oxide (HfO₂) powder were mixed at aratio of 4:96. The density of the manufactured target was about 95% ofthe theoretical value of 4(Y₂O₃)-96(HfO₂).

[Production of Ferroelectric Recording Medium]

The ferroelectric recording medium was manufactured according to thefollowing method. A non-doped monocrystalline silicon with a plane (001)was used as a substrate. The shape of the substrate was a disk shapehaving an opening portion at the center, and the substrate had an outerdiameter of 65 mm, an inner diameter of 20 mm, and a thickness of 0.8mm. The substrate in the disk shape was put into a deposition apparatus(by CANON ANELVA CORPORATION), and gold (Au) as an electrode layer wasdeposited at 30 nm on the surface of the substrate by using an RFsputtering method with a substrate temperature of 200° C. using Ar asthe sputter gas Ar at a pressure of 1 Pa. Subsequently, using an RFsputtering method, CeO₂ as a paraelectric layer was deposited at 30 nmwith a substrate temperature of 350° C. using Ar and O₂ (mixing ratio of3:1) as the sputter gas at a pressure of 1 Pa. Subsequently, using an RFsputtering method, Hf_(0.5)Zr_(0.5)O₂ as a ferroelectric layer wasdeposited at 30 nm with a substrate temperature of 400° C. using Ar andO₂ (mixing ratio of 3:1) as the sputter gas at a pressure of 1 Pa. Ontop of Hf_(0.5)Zr_(0.5)O₂ as the ferroelectric layer, a diamond-likecarbon (DLC) film as a protection layer was deposited at 5 nm by an ionbeam method with a substrate temperature of 150° C. The electrode layeris deposited on the entire surface of the substrate, but a width 10 mmof the inner circumferential portion was masked, so that the electrodelayer was not deposited on a portion around the opening portion in thecenter of the paraelectric layer, the ferroelectric layer, and theprotection layer. Finally, on the protection layer, a lubricant layerwas formed by applying a perfluoropolyether-based lubricant with a filmthickness of 1.5 nm using a dip method. As a result, a ferroelectricrecording medium was obtained.

Tables 1 to 4 illustrate the configuration of each layer constitutingthe ferroelectric recording medium.

[Evaluation of Properties of Ferroelectric Layer]

With respect to the ferroelectric layer, diffraction strength of the(111) plane measured by X-ray diffraction (XRD), smoothness, a densityof current leakage, and a density of current leakage due todeterioration over time were evaluated as the properties of theferroelectric layer of the ferroelectric recording medium.

(Evaluation of Diffraction Strength of (111) Plane of FerroelectricLayer, Measured by XRD)

After the electrode layer, the paraelectric layer, and the ferroelectriclayer were deposited according to the above-described [Production offerroelectric recording medium], the substrate was taken out from thedeposition apparatus. With respect to the substrate taken out, thediffraction strength of the (111) plane of Hf_(0.5)Zr_(0.5)O₂ wasmeasured using an x-ray diffraction (XRD) apparatus with the incidentX-ray being 0 and the detection angle being 20. As a result, thediffraction strength was 1200 cps. Table 5 illustrates a measurementresult.

(Evaluation of Smoothness of Ferroelectric Layer)

After the electrode layer, the paraelectric layer, and the ferroelectriclayer were deposited according to the above-described [Production offerroelectric recording medium], the substrate was taken out from thedeposition apparatus. A surface roughness (Ra) on the side of theferroelectric layer of the substrate taken out was measured, and thesurface roughness of the ferroelectric layer was evaluated on the basisof the evaluation criteria below. For the measurement, an atomic forcemicroscope (by BRUKER) was used.

((Evaluation Criteria))

A: The surface roughness of the substrate was less than 0.5 nm.

B: The surface roughness of the substrate was 0.5 nm to less than 1.0nm.

C: The surface roughness of the substrate was 1.0 nm or more.

(Evaluation of Density of Current Leakage in Ferroelectric Layer)

Similar to the above-described (Evaluation of smoothness offerroelectric layer), after the electrode layer, the paraelectric layer,and the ferroelectric layer were deposited according to theabove-described [Production of ferroelectric recording medium], thesubstrate was taken out from the deposition apparatus. An evaluationsample was manufactured by forming an Au electrode pad (a film thickness200 nm) having a size of 0.5 mm by 0.5 mm on surface of theferroelectric layer of the substrate taken out. The density of currentleakage between an electrode layer and an Au electrode pad of thisevaluation sample was measured. As a result, the density of currentleakage was about 5×10⁻⁶ A/cm² when 5 V was applied. Table 5 shows themeasurement result.

(Evaluation of Density of Current Leakage in Ferroelectric Layer Due toDeterioration Over Time)

A ferroelectric recording medium manufactured according to theabove-described [Production of ferroelectric recording medium] wasmaintained under an environment at a temperature of 80° C. and ahumidity of 80% for two weeks. After the ferroelectric recording mediumtaken out was dried, an evaluation sample was manufactured by forming Auelectrode pad (a film thickness 200 nm) having a size of 0.5 mm by 0.5mm at a position of a radius of 40 mm. This evaluation sample wasattached to the spindle shaft of the ferroelectric storage apparatus,and the density of current leakage between the spindle shaft and the Auelectrode pad of this evaluation sample was measured. As a result, thedensity of current leakage was about 5×10⁻⁶ A/cm² when 5 V was applied.Table 5 shows the measurement result.

The density of current leakage, due to deterioration over time, of theferroelectric recording medium according to Example 13 explained laterwas about 1×10⁻⁵ A/cm2 when 5 V was applied. Table 5 shows themeasurement result (see Example 13).

Table 5 illustrates evaluation results of each of the above-describedproperties of the ferroelectric layer.

[Manufacture of Ferroelectric Storage Apparatus]

(Manufacture of First Conductive Probe)

The first conductive probe was manufactured according to the followingmethod. Using a sputtering method, molybdenum was deposited at athickness of 1 μm on a quartz substrate having a thickness of 0.2 mm anda plane (0001). A photoresist pattern having an opening portion in anequilateral triangle with a side of 0.3 μm was formed on a molybdenumsurface by a photoresist method. Subsequently, by wet etching,molybdenum in a portion that is not covered by a photoresist pattern wasetched to a depth of about 0.3 μm. As an etching liquid, a mixed liquidincluding phosphoric acid (H₃PO₄), nitric acid (HNO₃), acetic acid(CH₃COOH), and water was used. Thereafter, molybdenum was deposited at athickness of 1 μm by a sputtering method. Thereafter, a chip of a firstconductive probe having a needle-shaped electrode of molybdenum wasformed on a quartz substrate by cutting a quartz-substrate including aprobe formation portion into a square with a size of 0.5 mm by 0.5 mmand thereafter removing the photoresist. The needle-shaped electrode ofthis chip was surrounded and covered by the molybdenum layer, and thetip portion of the needle slightly protruded from the molybdenum layer.

(Manufacture of First Probe Slider)

The first probe slider constituted by Al₂O₃—TiC(AlTiC) was manufactured.The outer shape of the first probe slider was such that the uppersurface was 2 mm×1.5 mm, the thickness was 0.5 mm, a width of a leadingend face that is an inflow end face of air flow was 0.2 mm, and arecessed portion of 0.2 mm for attaching the chip of the firstconductive probe was provided on an outflow end face. In addition, Auwires for the conductive probe and an Au electrode and wires forapplying a voltage to a piezoelectric element (quartz) were provided onan outflow end face.

(Manufacture of Second Conductive Probe and Second Probe Slider)

A monocrystalline silicon substrate (plane (001)) having a thickness of0.2 mm was heated to 550° C., and on the surface thereof, an electrodelayer (first electrode) of Au having a thickness of 200 nm, a PZT layer(electrostrictive element) having a thickness of 500 nm, an electrodelayer (second electrode) of Au having a thickness of 200 nm, a PZT layer(piezoelectric element) having a thickness of 500 nm, and an electrodelayer (third electrode) of Au having a thickness of 200 nm weredeposited by an RF sputtering method. In each electrode layer, a circuitpattern connected to an electrode layer outside of the laminatestructure was provided by photolithography. By a method similar to themethod used for the first conductive probe, a needle-shaped electrode ofmolybdenum was formed on these layers. Thus, the chip of the secondconductive probe was manufactured.

Using the chip of the second conductive probe, the second probe sliderwas manufactured by a method similar to the method used for the firstprobe slider. Wires connected to the first electrode, the secondelectrode, and the third electrode were provided in the second probeslider. In this case, the first electrode and the second electrode wereused for voltage application to the electrostrictive element, the secondelectrode and the third electrode were used for detection of an outputsignal from the piezoelectric element, and the third electrode was usedfor application of a write signal to the ferroelectric recording medium.

(Manufacture of Ferroelectric Recording Medium Driving Unit)

A spindle motor having a structure as illustrated in FIG. 26 wasmanufactured for rotation driving of the ferroelectric recording medium.An aluminum alloy was used as the housing, the shaft member was S45Chardened steel with a diameter of 3 mm, the shaft end portion was in aprotruding shape with a curvature radius of 6 mm Cylindrical50Cu-47Fe-3Sn sintered metal was used for the bearing sleeve, and flat50Cu-47Fe-3Sn sintered metal was used for the housing bottom portion.ISO VG100 was used for the lubricant oil, and a conductive carbon fiber(VGCF-H, Showa Denko K.K., fiber diameter of 150 nm) was added by 2% bymass.

(Manufacture of Probe Driving Unit)

As a probe driving unit, a probe driving unit having the structure asillustrated in FIG. 8 was manufactured. The probe driving unit wasmanufactured using a conventional HDD driving unit.

(Manufacture of Control Unit)

As a control unit, a control unit having a structure as illustrated inFIG. 8 was manufactured. The control unit was manufactured using agenerally-available power source and a control device.

(Manufacture of Recording-and-Reproduction Signal Processing Unit)

A recording-and-reproduction signal processing unit was manufactured. Awrite signal is generated by a bipolar power source for generating apositive or negative voltage corresponding to write information, andreading is performed by an amplifier for amplifying a weak tunnelcurrent flowing between the conductive probe and the ferroelectricrecording layer and an A/D conversion apparatus converting this intodigital data. A waveform generated by the bipolar power source was atriangle wave. In addition, a DC power source that causes thepiezoelectric element provided between the conductive probe and theheader slider to expand and shrink was also provided.

(Housing)

As a housing, a housing having a structure as illustrated in FIG. 8 wasmanufactured.

(Manufacture of Ferroelectric Storage Apparatus 1)

The ferroelectric storage apparatus 1 having the structure asillustrated in FIG. 8 was manufactured using the ferroelectric recordingmedium, the first conductive probe, the first probe slider, theferroelectric recording medium driving unit, the probe driving unit, thecontrol unit, the recording-and-reproduction signal processing unit, andthe housing that have been manufactured. The conductive probe wasmounted on the lower surface of the tip of the probe slider, this probeslider was attached to the suspension arm, and a structure for drivingthe surface of the ferroelectric recording medium with a voice coilmotor was provided. The housing of the ferroelectric storage apparatuswas sealed, and the inside was filled with argon gas at the atmosphericpressure. In addition, 1 g of silica gel was sealed in the housing as adesiccant.

(Manufacture of Ferroelectric Storage Apparatus 2)

A ferroelectric storage apparatus 2 was manufactured in the same manneras the above-described ferroelectric storage apparatus 1 wasmanufactured, except that the conductive probe and the probe slider ofthe ferroelectric storage apparatus 1 were replaced by a secondconductive probe and a second probe slider, respectively.

[Performance of Ferroelectric Storage Apparatus 1]

As the performance of the ferroelectric storage apparatus 1, arecording-and-reproduction test 1 of the ferroelectric storage apparatus1, a measurement of a tunnel current difference during reading, and ameasurement of the amount of charge of triboelectric charging wereconducted.

(Recording-and-Reproduction Examination)

Recording-and-reproduction test of manufactured ferroelectric storageapparatus 1 was conducted. The ferroelectric recording medium wasrotated at 5400 rpm, the probe slider is caused to travel by floatingabove the ferroelectric recording medium surface, and the position ofthe probe slider was fixed to a track position at a radius of 40 mm ofthe ferroelectric recording medium. Thereafter, the conductive probe wasswitched to an information read circuit, a bias voltage of +500 mV wasapplied across the ferroelectric recording medium and the conductiveprobe, and a tunnel current from the conductive probe was monitored.Then, by applying a DC voltage to the piezoelectric element, theconductive probe was gradually brought closer to the ferroelectricrecording medium, and the voltage applied to the piezoelectric elementwas fixed at the position where the average value of the tunnel currentbecomes 2 pA.

Thereafter, the conductive probe was switched to the information writecircuit, and information of 255 sectors is written to this trackposition. Each sector is constituted by a data area and a servoinformation area, and the servo information area is constituted by aburst information area and an address information area. Writing ofinformation was performed with a triangle wave of which the peak voltageis ±5V, and the write frequency was 2.3 GHz. Information is writtenonce, and the write time was about 10 milliseconds. This is the lengthof a single bit, i.e., 10 nm, in the circumferential direction on thesurface of the ferroelectric recording medium.

After information was written to the ferroelectric recording medium, theconductive probe was switched to the information read circuit, and thetunnel current was monitored. At this occasion, the voltage applied tothe piezoelectric element was adjusted so as to make the SNR 3 dB ormore. The amplitude of the tunnel current was about 3 pA, and the lengthof time from the end of writing of information to the ferroelectricrecording medium to reading was about 0.1 milliseconds.

According to the above method, it was confirmed that information can bewritten to the ferroelectric storage apparatus 1 and written informationcan be read.

When the above-described reading of information was repeated 10 times,the SNR during reading of information became less than 3 dB, andtherefore, the same data was rewritten to the same track position. As aresult, the SNR during reading of information recovered to 3 dB or more.

(Measurement 1 of Tunnel Current Difference During Reading)

When information is read from the ferroelectric recording medium, atunnel current difference between a positively charged bit of theferroelectric layer and a negatively charged bit of the ferroelectriclayer was measured. The bias voltage across the ferroelectric recordingmedium and the conductive probe was 500 mV. On the surface layer side ofthe ferroelectric layer, the tunnel current of the positively chargedbit decreases, and the tunnel current of the negatively charged bitincreases. In this case, specifically, a tunnel current from thenegatively charged bit was measured, when the distance between theconductive probe and the ferroelectric recording medium was adjusted sothat the tunnel current of the positively charged bit on the surfacelayer side became 1 pA. As a result, the tunnel current from thenegatively charged bit was about 3 pA, and a tunnel current differencetherebetween was about 2 pA. Table 6 shows the result.

(Measurement 2 of Tunnel Current Difference During Reading)

When information is read from the ferroelectric recording medium, a biasvoltage of ±500 mV was applied across the ferroelectric recording mediumand the conductive probe. The bias voltage was a square wave of 1 GHz,and the read speed was 500 Mbit/second.

The distance between the conductive probe and the ferroelectricrecording medium was adjusted so as to make the tunnel current of thepositively charged bit on the surface layer side of the ferroelectriclayer 5 pA on average when the bias voltage to the conductive probe was−500 mV. In this case, when the bias voltage to the conductive probe was+500 mV, the average value of the tunnel current has decreased to 1 pA.This is considered to be because that there is a rectifying effect inthe bonding portion between the ferroelectric layer and the paraelectriclayer. When the negatively charged bit on the surface layer side of theferroelectric layer was evaluated in the same manner, the tunnel currentin the forward direction was 25 pA on average, and the tunnel current inthe reverse direction was 5 pA. According to the above-describedmeasurement, in a case where the bias voltage to the conductive probewas changed to positive and negative, the tunnel current differencebetween the positively charged bit and the negatively charged bit was 20pA at most. Table 6 illustrates the result.

(The Amount of Charge of Triboelectric Charging)

The static electricity of the ferroelectric storage apparatus(specifically, the ferroelectric storage apparatus the inside of whichwas filled with argon gas at the atmospheric pressure and of which thehousing is sealed with a desiccant placed therein) manufacturedaccording to the above-described “Manufacture of ferroelectric storageapparatus” was removed. Thereafter, the ferroelectric recording mediumwas rotated at 5600 rpm, and the probe slider performed seek operationsonce in every 4 Hz (an operation for returning to the innermostcircumference after performing a seek operation to the outermostcircumference from the innermost circumference is defined as one cycle).Thereafter, the seek operation was stopped, and the amount of chargebetween the housing and the conductive probe was measured at one secondafter the stop of the seek operation. As a result, the amount of chargeof triboelectric charging was 0.1 nC.

As a reference ferroelectric storage apparatus, a first referenceferroelectric storage apparatus (Example 11) of which the inside isfilled with nitrogen gas at atmospheric pressure is filled and of whichthe housing is sealed with a desiccant placed therein was manufactured,and a second reference ferroelectric storage apparatus (Example 12) inthe inside of which a desiccant is placed and the inside of which isbrought to the atmosphere through a filter was manufactured. Withrespect to these reference ferroelectric storage apparatuses, the amountof charge is measured in the same manner as the measurement of theferroelectric storage apparatus according to the Example 1. As a result,in the first reference ferroelectric storage apparatus, the amount ofcharge of triboelectric charging was 3 nC, and in the second referenceferroelectric storage apparatus, the amount of charge of triboelectriccharging was 4 nC.

Therefore, with respect to the ferroelectric storage apparatus of theExample 1, it was confirmed that the triboelectric charging wasalleviated due to the argon gas filling the housing.

[Performance of Ferroelectric Storage Apparatus 2]

As the performance of the ferroelectric storage apparatus 2, for readingof information from the ferroelectric recording medium, reading using anatomic force was performed. The reading using the atomic force wasperformed by measuring an amplitude of a signal of read informationobtained from a piezoelectric element during therecording-and-reproduction test of the ferroelectric storage apparatus2.

(Measurement 3 of Amplitude of Signal of Read Information Obtained fromPiezoelectric Element when Reading is Performed Using Atomic Force)

Recording-and-reproduction test of manufactured ferroelectric storageapparatus was conducted. The ferroelectric recording medium was 5400rpm, the probe slider was caused to travel by floating above theferroelectric recording medium surface, and the position of the probeslider was fixed to a track position at a radius of 40 mm of theferroelectric recording medium. Thereafter, while the output voltagefrom the piezoelectric element across the second electrode and the thirdelectrode was monitored, a DC voltage applied to the electrostrictiveelement across the first electrode and the second electrode is graduallyincreased, so that the conductive probe is brought closer to theferroelectric recording medium, and the voltage applied to theelectrostrictive element was controlled so that the average value of theoutput voltage from the piezoelectric element became about +5 μV.

In this state, an information write signal was applied from the thirdelectrode to the conductive probe, and information of 255 sectors iswritten to this track position. Each sector is constituted by a dataarea and a servo information area, and the servo information area isconstituted by a burst information area and an address information area.Writing of information was performed with a triangle wave of which thepeak voltage is ±5V, and the write frequency was 2.3 GHz. Information iswritten once, and the write time was about 10 milliseconds. This is thelength of a single bit, i.e., 10 nm, in the circumferential direction onthe surface of the ferroelectric recording medium.

After information was written to the ferroelectric recording medium,while the output voltage from the piezoelectric element was monitored,the voltage applied to the piezoelectric element was adjusted so as tomake the SNR of the output voltage from the piezoelectric element 3 dBor more. As a result, information of 255 sectors that are the same asthe written information were read from the ferroelectric recordingmedium. The amplitude of the signal of read information from thepiezoelectric element was about 3 μV, and the length of time from theend of writing of information to the ferroelectric recording medium toreading was about 0.1 milliseconds. Table 6 illustrates a measurementresult of amplitudes of signals of read information obtained from thepiezoelectric elements.

Examples 2 to 6

Ferroelectric recording media according to Examples 2 to 6 weremanufactured in the same manner as in the Example 1 explained in theabove-described [Production of ferroelectric recording medium] exceptthat the materials included in the electrode layers were changed to Ge,Pb, Al, Cu, and Cr as shown in Table 2. With respect to each Example, atest of a diffraction strength of (111) plane of Hf_(0.5)Zr_(0.5)O₂ ofthe ferroelectric layer and a recording-and-reproduction test of theferroelectric storage apparatus were conducted. Table 1 to Table 4 showconfigurations of the layers constituting each of the ferroelectricrecording media, Table 5 shows formation conditions of ferroelectriclayers and evaluation results of properties, and Table 6 shows testresults of dielectric storage apparatuses.

Examples 7 to 9

Ferroelectric recording media according to Examples 7 to 9 weremanufactured in the same manner as in the Example 1 explained in theabove-described [Production of ferroelectric recording medium] exceptthat the materials included in the paraelectric layers were changed to10(Y₂O₃)-90(ZrO₂), Al₂O₃, and TiO₂ as shown in Table 3. With respect toeach Example, a test of a diffraction strength of (111) plane ofHf_(0.5)Zr_(0.5)O₂ of the ferroelectric layer and arecording-and-reproduction test of the ferroelectric storage apparatuswere conducted. Table 1 to Table 4 show configurations of the layersconstituting each of the ferroelectric recording media, Table 5 showsformation conditions of ferroelectric layers and evaluation results ofproperties, and Table 6 shows test results of dielectric storageapparatuses.

Example 10

A ferroelectric recording medium according to Example 10 wasmanufactured in the same manner as in the Example 1 explained in theabove-described [Production of ferroelectric recording medium] exceptthat the paraelectric layer was not provided. With respect to eachExample, a test of a diffraction strength of (111) plane ofHf_(0.5)Zr_(0.5)O₂ of the ferroelectric layer, arecording-and-reproduction test of the ferroelectric storage apparatus,and a measurement of the amount of charge of triboelectric charging wereconducted. In this Example 10, the density of current leakage betweenthe electrode layer and the Au electrode pad of the substrate on whichthe electrode layer, the paraelectric layer, and the ferroelectric layerwere deposited was about 1×10⁻⁵ A/cm² when 5 V was applied. Table 1 toTable 4 show configurations of the layers constituting each of theferroelectric recording media, Table 5 shows formation conditions offerroelectric layers and evaluation results of properties, and Table 6shows test results of dielectric storage apparatuses.

Examples 11 and 12

Ferroelectric recording media according to Examples 11 and weremanufactured in the same manner as in the Example 1 explained in theabove-described [Production of ferroelectric recording medium] exceptthat the inside was filled with nitrogen gas or atmosphere of theatmospheric pressure and that the housing was sealed with a desiccantplaced therein. Table 1 to Table 4 show configurations of the layersconstituting each of the ferroelectric recording media, Table 5 showsformation conditions of ferroelectric layers and evaluation results ofproperties, and Table 6 shows test results of dielectric storageapparatuses.

Example 13

A ferroelectric recording medium according to Example 13 wasmanufactured in the same manner as in the Example 1 explained in theabove-described [Production of ferroelectric recording medium] exceptthat deposition was performed without masking a width 10 mm of the innercircumferential portion of the substrate, and the ferroelectricrecording medium according to Example 13 having the electrode layer, theparaelectric layer, the ferroelectric layer, and the protection layerdeposited on the entire surface of the substrate was manufactured. Table1 to Table 4 show configurations of the layers constituting each of theferroelectric recording media, Table 5 shows formation conditions offerroelectric layers and evaluation results of properties, and Table 6shows test results of dielectric storage apparatuses.

Example 14 and Comparative Examples 1 to 4

Ferroelectric recording media according to Example 14 and ComparativeExamples 1 to 4 were manufactured in the same manner as in the Example 1explained in the above-described [Production of ferroelectric recordingmedium] except that the material used for the substrate was changed toan A-plane sapphire substrate (Example 14), an electroless NiP-plated5000-series aluminum alloy substrate (Comparative Example 1), anamorphous glass substrate (Comparative Example 2), an MgO substrate of(100) plane (Comparative Example 3), and a C-plane sapphire substrate(Comparative Example 4) as shown in Table 1, and that, in theComparative Example 1, the electrode layer was not provided. Withrespect to the Example 14 and the Comparative Examples 1 to 4, a test ofa diffraction strength of (111) plane of Hf_(0.5)Zr_(0.5)O₂ of theferroelectric layer of and a recording-and-reproduction test of theferroelectric storage apparatus were conducted. Table 1 to Table 4 showconfigurations of the layers constituting each of the ferroelectricrecording media, Table 5 shows formation conditions of ferroelectriclayers and evaluation results of properties, and Table 6 shows testresults of dielectric storage apparatuses.

Example 15

A ferroelectric recording medium according to Example 15 wasmanufactured in the same manner as in the Example 1 explained in theabove-described [Production of ferroelectric recording medium] exceptthat the substrate temperature during deposition of the ferroelectriclayer was reduced by 80° C. to 320° C. Table 1 to Table 4 showconfigurations of the layers constituting each of the ferroelectricrecording media, Table 5 shows formation conditions of ferroelectriclayers and evaluation results of properties, and Table 6 shows testresults of dielectric storage apparatuses.

Example 16

A ferroelectric recording medium according to Example 16 wasmanufactured in the same manner as in the Example 1 explained in theabove-described [Production of ferroelectric recording medium] exceptthat the ferroelectric recording medium, according to Example 16 has theconfiguration of Example 5 and that the substrate temperature duringdeposition of the ferroelectric layer was reduced by 80° C. to 320° C.Table 1 to Table 4 show configurations of the layers constituting eachof the ferroelectric recording media, Table 5 shows formation conditionsof ferroelectric layers and evaluation results of properties, and Table6 shows test results of dielectric storage apparatuses.

Example 17

A ferroelectric recording medium according to Example 17 wasmanufactured in the same manner as in the Example 1 explained in theabove-described [Production of ferroelectric recording medium] exceptthat the ferroelectric recording medium according to Example 17 has theconfiguration of Example 6 and that the substrate temperature duringdeposition of the ferroelectric layer was reduced by 80° C. to 320° C.Table 1 to Table 4 show configurations of the layers constituting eachof the ferroelectric recording media, Table 5 shows formation conditionsof ferroelectric layers and evaluation results of properties, and Table6 shows test results of dielectric storage apparatuses.

In all of the Examples 15 to 17, halo was observed in XRD (111)diffraction patterns, and the diffraction strength decreased, but whenthe substrate was heated to 520° C. (+200° C. with reference to thedeposition temperature), the halo pattern changed to a signal having asharp peak, and the diffraction strengths became 1800 (Example 15), 1600(Example 16), and 1600 (Example 17). As a result, based on the electronmicroscope observation and the electron diffraction result, it isassumed that the ferroelectric layers according to Examples 15 to 17were made into an amorphous structure with short-range order of whichthe length, the width, and the height were 2 nm or less. Therefore, whenExamples 15 to 17 are compared with Examples 1, 5, and 6, the density ofcurrent leakage did not change, but the smoothness on the growth surfaceof the ferroelectric layer has improved in any of the Examples 15 to 17.

Example 18 to 23

Ferroelectric recording media according to Examples 18 to 23 weremanufactured in the same manner as in the Example 1 explained in theabove-described [Production of ferroelectric recording medium] exceptthat the film thicknesses of the paraelectric layer and theferroelectric layer were changed to values as shown in Table 3 and Table4.

TABLE 1 Substrate Difference in Lattice Constant Lattice fromFerroelectric Constant Material Crystal System and Structure Layer [%][Å] Example 1 Si(001) Cubic Crystal System 4 5.4 Diamond StructureExample 2 Si(001) Cubic Crystal System 4 5.4 Diamond Structure Example 3Si(001) Cubic Crystal System 4 5.4 Diamond Structure Example 4 Si(001)Cubic Crystal System 4 5.4 Diamond Structure Example 5 Si(001) CubicCrystal System 4 5.4 Diamond Structure Example 6 Si(001) Cubic CrystalSystem 4 5.4 Diamond Structure Example 7 Si(001) Cubic Crystal System 45.4 Diamond Structure Example 8 Si(001) Cubic Crystal System 4 5.4Diamond Structure Example 9 Si(001) Cubic Crystal System 4 5.4 DiamondStructure Example 10 Si(001) Cubic Crystal System 4 5.4 DiamondStructure Example 11 Si(001) Cubic Crystal System Diamond Structure 45.4 Example 12 Si(001) Cubic Crystal System Diamond Structure 4 5.4Example 13 Si(001) Cubic Crystal System 4 5.4 Diamond Structure Example14 A-plane Hexagonal Crystal System 8 4.8 Sapphire Corundum TypeStructure Example 15 Si(001) Cubic Crystal System 4 5.4 DiamondStructure Example 16 Si(001) Cubic Crystal System 4 5.4 DiamondStructure Example 17 Si(001) Cubic Crystal System 4 5.4 DiamondStructure Example 18 Si(001) Cubic Crystal System 4 5.4 DiamondStructure Example 19 Si(001) Cubic Crystal System 4 5.4 DiamondStructure Example 20 Si(001) Cubic Crystal System 4 5.4 DiamondStructure Example 21 Si(001) Cubic Crystal System 4 5.4 DiamondStructure Example 22 Si(001) Cubic Crystal System 4 5.4 DiamondStructure Example 23 Si(001) Cubic Crystal System 4 5.4 DiamondStructure Comparative NiP-plated Amorphous Structure (NiP Film) — —Example 1 5000 Series Aluminum Alloy Comparative Amorphous AmorphousStructure (NiP Film) — — Example 2 Glass Comparative MgO(100) CubicCrystal System 19  4.2 Example 3 Sodium Chloride Type StructureComparative C-plane Hexagonal Crystal System 150   13   Example 4Sapphire Corundum Type Structure

TABLE 2 Electrode Layer Difference in Lattice Constant Lattice fromFerroelectric Constant Material Crystal System and Structure Layer [%][Å] Example 1 Au Cubic Crystal System, Face- 21 4.1 Centered CubicLattice Structure Example 2 Ge Cubic Crystal System, Diamond 10 5.7Structure Example 3 Pb Cubic Crystal System, Face-  4 5.0 Centered CubicLattice Structure Example 4 Al Cubic Crystal System, Face- 23 4.0Centered Cubic Lattice Structure Example 5 Cu Cubic Crystal System,Face- 31 3.6 Centered Cubic Lattice Structure Example 6 Cr Cubic CrystalSystem, Body- 44 2.9 Centered Cubic Lattice Structure Example 7 Ge CubicCrystal System, Diamond 10 5.7 Structure Example 8 Ge Cubic CrystalSystem, Diamond 10 5.7 Structure Example 9 Ge Cubic Crystal System,Diamond 10 5.7 Structure Example 10 Au Cubic Crystal System, Face- 214.1 Centered Cubic Lattice Structure Example 11 Au Cubic Crystal System,Face- 21 4.1 Centered Cubic Lattice Structure Example 12 Au CubicCrystal System, Face- 21 4.1 Centered Cubic Lattice Structure Example 13Au Cubic Crystal System, Face- 21 4.1 Centered Cubic Lattice StructureExample 14 Au Cubic Crystal System, Face- 21 4.1 Centered Cubic LatticeStructure Example 15 Au Cubic Crystal System, Face- 21 4.1 CenteredCubic Lattice Structure Example 16 Cu Cubic Crystal System, Face- 31 3.6Centered Cubic Lattice Structure Example 17 Cr Cubic Crystal System,Body- 44 2.9 Centered Cubic Lattice Structure Example 18 Au CubicCrystal System, Face- 21 4.1 Centered Cubic Lattice Structure Example 19Au Cubic Crystal System, Face- 21 4.1 Centered Cubic Lattice StructureExample 20 Au Cubic Crystal System, Face- 21 4.1 Centered Cubic LatticeStructure Example 21 Au Cubic Crystal System, Face- 21 4.1 CenteredCubic Lattice Structure Example 22 Au Cubic Crystal System, Face- 21 4.1Centered Cubic Lattice Structure Example 23 Au Cubic Crystal System,Face- 21 4.1 Centered Cubic Lattice Structure Comparative — — — —Example 1 Comparative Au Cubic Crystal System, Face- 21 4,1 Example 2Centered Cubic Lattice Structure Comparative Au Cubic Crystal System,Face- 21 4.1 Example 3 Centered Cubic Lattice Structure Comparative AuCubic Crystal System, Face- 21 4.1 Example 4 Centered Cubic LatticeStructure

TABLE 3 Paraelectric Layer Difference in Lattice Constant Lattice Filmfrom Ferroelectric Constant Thickness Material Crystal System .Structure Layer [%] [Å] [nm] Example 1 CeO₂ Cubic Crystal System, 4 5.430 Fluorite Type Structure Example 2 CeO₂ Cubic Crystal System, 4 5.4 30Fluorite Type Structure Example 3 CeO₂ Cubic Crystal System, 4 5.4 30Fluorite Type Structure Example 4 CeO₂ Cubic Crystal System, 4 5.4 30Fluorite Type Structure Example 5 CeO₂ Cubic Crystal System, 4 5.4 30Fluorite Type Structure Example 6 CeO₂ Cubic Crystal System, 4 5.4 30Fluorite Type Structure Example 7 10(Y₂O₃)- Cubic Crystal System, 2 5.130 90(ZrO₂) Fluorite Type Structure Example 8 Al₂O₃ Trigonal CrystalSystem, 8 4.8 30 Corundum Type Structure Example 9 TiO₂ TetragonalCrystal System, 12  4.6 30 Rutile Type Structure Example 10 — — — — —Example 11 CeO₂ Cubic Crystal System, 4 5.4 30 Fluorite Type StructureExample 12 CeO₂ Cubic Crystal System, 4 5.4 30 Fluorite Type StructureExample 13 CeO₂ Cubic Crystal System, 4 5.4 30 Fluorite Type StructureExample 14 CeO₂ Cubic Crystal System, 4 5.4 30 Fluorite Type StructureExample 15 CeO₂ Cubic Crystal System, 4 5.4 30 Fluorite Type StructureExample 16 CeO₂ Cubic Crystal System, 4 5.4 30 Fluorite Type StructureExample 17 CeO₂ Cubic Crystal System, 4 5.4 30 Fluorite Type StructureExample 18 CeO₂ Cubic Crystal System, 4 5.4 30 Fluorite Type StructureExample 19 CeO₂ Cubic Crystal System, 4 5.4 20 Fluorite Type StructureExample 20 CeO₂ Cubic Crystal System, 4 5.4 10 Fluorite Type StructureExample 21 CeO₂ Cubic Crystal System, 4 5.4  5 Fluorite Type StructureExample 22 CeO₂ Cubic Crystal System, 4 5.4  1 Fluorite Type StructureExample 23 CeO₂ Cubic Crystal System, 4 5.4  1 Fluorite Type StructureComparative CeO₂ Cubic Crystal System, 4 5.4 30 Example 1 Fluorite TypeStructure Comparative CeO₂ Cubic Crystal System, 4 5.4 30 Example 2Fluorite Type Structure Comparative CeO₂ Cubic Crystal System, 4 5.4 30Example 3 Fluorite Type Structure Comparative CeO₂ Cubic Crystal System,4 5.4 30 Example 4 Fluorite Type Structure

TABLE 4 Ferroelectric Layer Lattice Film Constant Thickness MaterialCrystal System and Structure [Å] (nm) Example 1 Hf_(0.5)Zr_(0.5)O₂Orthorhombic Crystal System(*). 5.1-5.3 30 Fluorite Type StructureExample 2 Hf_(0.5)Zr_(0.5)O₂ Orthorhombic Crystal System(*). 5.1-5.3 30Fluorite Type Structure Example 3 Hf_(0.5)Zr_(0.5)O₂ OrthorhombicCrystal System(*). 5.1-5.3 30 Fluorite Type Structure Example 4Hf_(0.5)Zr_(0.5)O₂ Orthorhombic Crystal System(*). 5.1-5.3 30 FluoriteType Structure Example 5 Hf_(0.5)Zr_(0.5)O₂ Orthorhombic CrystalSystem(*). 5.1-5.3 30 Fluorite Type Structure Example 6Hf_(0.5)Zr_(0.5)O₂ Orthorhombic Crystal System(*). 5.1-5.3 30 FluoriteType Structure Example 7 Hf_(0.5)Zr_(0.5)O₂ Orthorhombic CrystalSystem(*). 5.1-5.3 30 Fluorite Type Structure Example 8Hf_(0.5)Zr_(0.5)O₂ Orthorhombic Crystal System(*). 5.1-5.3 30 FluoriteType Structure Example 9 Hf_(0.5)Zr_(0.5)O₂ Orthorhombic CrystalSystem(*). 5.1-5.3 30 Fluorite Type Structure Example 10Hf_(0.5)Zr_(0.5)O₂ Orthorhombic Crystal System(*). 5.1-5.3 30 FluoriteType Structure Example 11 Hf_(0.5)Zr_(0.5)O₂ Orthorhombic CrystalSystem(*). 5.1-5.3 30 Fluorite Type Structure Example 12Hf_(0.5)Zr_(0.5)O₂ Orthorhombic Crystal System(*). 5.1-5.3 30 FluoriteType Structure Example 13 Hf_(0.5)Zr_(0.5)O₂ Orthorhombic CrystalSystem(*). 5.1-5.3 30 Fluorite Type Structure Example 14Hf_(0.5)Zr_(0.5)O₂ Orthorhombic Crystal System(*). 5.1-5.3 30 FluoriteType Structure Example 15 Hf_(0.5)Zr_(0.5)O₂ Orthorhombic CrystalSystem(*). 5.1-5.3 30 Fluorite Type Structure Example 16Hf_(0.5)Zr_(0.5)O₂ Orthorhombic Crystal System(*). 5.1-5.3 30 FluoriteType Structure Example 17 Hf_(0.5)Zr_(0.5)O₂ Orthorhombic CrystalSystem(*). 5.1-5.3 30 Fluorite Type Structure Example 18Hf_(0.5)Zr_(0.5)O₂ Orthorhombic Crystal System(*). 5.1-5.3 40 FluoriteType Structure Example 19 Hf_(0.5)Zr_(0.5)O₂ Orthorhombic CrystalSystem(*). 5.1-5.3 20 Fluorite Type Structure Example 20Hf_(0.5)Zr_(0.5)O₂ Orthorhombic Crystal System(*). 5.1-5.3 10 FluoriteType Structure Example 21 Hf_(0.5)Zr_(0.5)O₂ Orthorhombic CrystalSystem(*). 5.1-5.3  5 Fluorite Type Structure Example 22Hf_(0.5)Zr_(0.5)O₂ Orthorhombic Crystal System(*). 5.1-5.3  1 FluoriteType Structure Example 23 Hf_(0.5)Zr_(0.5)O₂ Orthorhombic CrystalSystem(*). 5.1-5.3  5 Fluorite Type Structure ComparativeHf_(0.5)Zr_(0.5)O₂ Orthorhombic Crystal System(*). 5.1-5.3 30 Example 1Fluorite Type Structure Comparative Hf_(0.5)Zr_(0.5)O₂ OrthorhombicCrystal System(*). 5.1-5.3 30 Example 2 Fluorite Type StructureComparative Hf_(0.5)Zr_(0.5)O₂ Orthorhombic Crystal System(*). 5.1-5.330 Example 3 Fluorite Type Structure Comparative Hf_(0.5)Zr_(0.5)O₂Orthorhombic Crystal System(*). 5.1-5.3 30 Example 4 Fluorite TypeStructure (*)Stable phase is monoclinic, tetragonal, or cubic crystalsystems.)

TABLE 5 Ferroelectric Layer Substrate Density of Current TemperatureDiffraction Density of Leakage due to during Intensity of CurentDeterioration Deposition (111) Plane Leakage Over Time [° C.] (Any givenvalue) Smoothness [A/cm²] [A/cm²] Example 1 400 1200 A 5 × 10⁻⁶ 5 × 10⁻⁶Example 2 400 1450 A 5 × 10⁻⁶ — Example 3 400 1500 A 5 × 10⁻⁶ — Example4 400 1000 A 5 × 10⁻⁶ — Example 5 400  800 B 1 × 10⁻⁵ — Example 6 400 700 B 1 × 10⁻⁵ — Example 7 400 1450 A 5 × 10⁻⁶ — Example 8 400 1000 A 5× 10⁻⁶ — Example 9 400  900 A 1 × 10⁻⁵ — Example 10 400 1100 A 1 × 10⁻⁵— Example 11 400 1200 A 5 × 10⁻⁶ — Example 12 400 1200 A 5 × 10⁻⁶ —Example 13 400 1200 A 5 × 10⁻⁶ 1 × 10⁻⁵ Example 14 400  500 B 1 × 10⁻⁵ —Example 15 320  900 A 5 × 10⁻⁶ — Example 16 320  600 A 1 × 10⁻⁵ —Example 17 320  600 A 1 × 10⁻⁵ — Example 18 400 1200 A 5 × 10⁻⁶ 5 × 10⁻⁶Example 19 400 1200 A 5 × 10⁻⁶ 5 × 10⁻⁶ Example 20 400 1200 A 5 × 10⁻⁶ 5× 10⁻⁶ Example 21 400 1200 A 5 × 10⁻⁶ 5 × 10⁻⁶ Example 22 400 1200 A 5 ×10⁻⁶ 5 × 10⁻⁶ Example 23 400 1200 A 5 × 10⁻⁶ 5 × 10⁻⁶ Comparative 400   0 C 1 × 10⁻⁴ — Example 1 Comparative 400    0 C 1 × 10⁻⁴ — Example 2Comparative 400  400 B 1 × 10⁻⁴ — Example 3 Comparative 400  200 C 1 ×10⁻⁴ — Example 4

TABLE 6 Ferroelectric Storage Device Measurement 1 Measurement 2Measurement 3 Amount of (Tunnel (Tunnel (Amplitude of Charge ofRecording Current Current Signal from Triboelectric and DifferenceDifference piezoelectric Charging Filling Gas Reproduction [pA]) [pA])element [μV]) [nC] Example 1 Argon Gas Successful 2 20 3 0.1 Example 2Argon Gas Successful 3 20 3 0.1 Example 3 Argon Gas Successful 3 20 30.1 Example 4 Argon Gas Successful 2 20 3 0.1 Example 5 Argon GasSuccessful 1 10 2 0.1 Example 6 Argon Gas Successful 1 10 2 0.1 Example7 Argon Gas Successful 3 20 3 0.1 Example 8 Argon Gas Successful 2 20 30.1 Example 9 Argon Gas Successful 2 20 3 0.1 Example 10 Argon GasSuccessful 1 5 2 0.1 Example 11 Nitrogen Gas Successful 1 5 2 3.0Example 12 Air Successful 1 5 2 4.0 Example 13 Argon Gas Successful 1 52 0.1 Example 14 Argon Gas Successful 1 5 2 0.1 Example 15 Argon GasSuccessful 1 5 2 0.1 Example 16 Argon Gas Successful 1 5 2 0.1 Example17 Argon Gas Successful 1 5 2 0.1 Example 18 Argon Gas Successful 1 10 20.1 Example 19 Argon Gas Successful 4 30 2 0.1 Example 20 Argon GasSuccessful 10 50 2 0.1 Example 21 Argon Gas Successful 10 50 2 0.1Example 22 Argon Gas Successful 6 40 2 0.1 Example 23 Argon GasSuccessful 12 70 2 0.1 Comparative Argon Gas Unsuccessful 0 0 — —Example 1 Comparative Argon Gas Unsuccessful 0 0 — — Example 2Comparative Argon Gas Unsuccessful 0 0 — — Example 3 Comparative ArgonGas Unsuccessful 0 0 — — Example 4

As shown in Table 6, it was confirmed that, in Examples 1, 2-1 to 2-3,and 3 to 23, information can be read, and there is a correlation betweenthe atomic force and the tunnel current obtained from the ferroelectricrecording medium during reading.

Therefore, according to a method for applying a voltage to theelectrostrictive element, information can be written to theferroelectric storage apparatus, and written information can be read.

Examples 2-1 to 2-3

Ferroelectric recording media according to Examples 2-1 to 2-3 weremanufactured in the same manner as in the Example 1 explained in theabove-described [Production of ferroelectric recording medium] exceptthat the ferroelectric recording medium according to Examples 2-1 to 2-3have the configuration of Example and that the ferroelectric recordingmedium according to Examples 2-1 to 2-3 had the compositions of theferroelectric layers deposited by the deposition methods as shown inTable 7. Specifically, the electrode layer, the paraelectric layer, andthe ferroelectric layer were deposited in the same manner as Example 2,the substrate was taken out from the deposition apparatus, and adiffraction strength of (111) plane of hafnium oxide of theferroelectric layer was measured with respect to each Example. Using themanufactured ferroelectric layers, ferroelectric storage apparatuseswere manufactured in the same manner as the Example 1, and arecording-and-reproduction test of the manufactured ferroelectricstorage apparatuses and a measurement of a tunnel current differenceduring reading were performed. Table 7 shows test results.

In all of the Examples 2-1 to 2-3, the substrate temperature duringdeposition was 400° C. In the microwave plasma MOCVD method, as sourcegases, tetrakis (ethylmethylamide) hafnium was used as a hafnium source,and tetrakis (ethylmethylamide) zirconium was used as a zirconiumsource, and oxygen and argon were added with a ratio of 1:1. Thereaction pressure was 100 Pa, and the input power was 800 W (2.45 GHz).In the MOCVD method, microwaves were not applied, and the substrateswere heated only. With respect to the substrates taken out, XRD θ-20scan was performed, and the diffraction strength of (111) plane ofhafnium oxide was measured. Table 7 shows the measurement results.

TABLE 7 Ferroelectric Layer Ferroelectric Storage Device DiffractionMeasurement 1 Measurement 2 Measurement 3 Intensity of (Tunnel (Tunnel(Amplitude of Crystal Lattice (111) Plane Recording Current CurrentSignal from System and Constant Deposition (Any given and DifferenceDifference piezoelectric Material Structure [Å] Method value)Reproduction [pA]) [pA]) element [μV]) Example 2 Hf_(0.5)Zr_(0.5)O₂Orthorhombic 5.1-5.3 RF Sputter 1450 Successful 3 20 3 Crystal SystemMethod (*), Fluorite Type Structure Example 4(Y₂O₃)- Orthorhombic5.1-5.3 RF Sputter 1300 Successful 3 20 3 2-1 96(HfO₂) Crystal SystemMethod (*), Fluorite Type Structure Example Hf_(0.5)Zr_(0.5)O₂Orthorhombic 5.1-5.3 Microwave 1400 Successful 3 20 3 2-2 Crystal SystemPlasma (*), Fluorite MOCVD Type Structure Method ExampleHf_(0.5)Zr_(0.5)O₂ Orthorhombic 5.1-5.3 MOCVD 800 Successful 1 10 3 2-3Crystal System Method (*), Fluorite Type Structure (*) Stable phase is amonoclinic, tetragonal, or cubic crystal systems.

As shown in Table 7, it was confirmed that when the reaction fieldduring deposition was assisted by plasma, the crystallinity of theferroelectric layer was improved.

As shown in Table 7, it was confirmed that, in the Examples 2-1 to 2-3,information can be read, and there is a correlation between the atomicforce and the tunnel current obtained from the ferroelectric recordingmedium during reading, similar to the Example 1. Therefore, in theExamples 2-1 to 2-3, according to a method for applying a voltage to theelectrostrictive element, information can be written to theferroelectric storage apparatus, and Written information can be read.

Although the embodiment has been hereinabove explained, theabove-described embodiment is presented as an example, and the presentinvention is not limited by the above-described embodiment. Theabove-described embodiment can be carried out in various other forms,and various combinations, omissions, replacements, changes, and the likecan be made without departing from the subject matter of the invention.These embodiments and modifications are included in the subject matterof the invention, and are included in the invention described in claimsand the scope equivalent thereto.

DESCRIPTION OF SYMBOLS

-   10 ferroelectric recording medium.-   11 substrate-   12 electrode layer-   13 ferroelectric recording layer-   131 ferroelectric layer-   131A data area-   131B servo information area-   131B-4 reference signal information-   132 paraelectric layer-   14 protection layer-   15 lubricant layer-   100 ferroelectric storage apparatus-   23 probe slider-   26, 26A, 26B, 26C conductive probe-   261 base body-   261 a recessed portion-   261 b, 263 b surface (principal surface)-   262 needle-shaped electrode-   263 insulating layer-   263 a through hole-   27 piezoelectric element-   27A first piezoelectric element-   27B second piezoelectric element-   30 ferroelectric recording medium driving unit-   31 housing (bearing cylinder)-   33 shaft member (spindle shaft)-   332 groove portion-   34 housing bottom portion-   40 probe driving unit-   50 recording-and-reproduction signal processing unit-   60 housing-   83 metal-   320 external storage apparatus-   321 a storage surface-   322 storage apparatus-   323 read device-   324 driving unit-   324A first driving unit-   324B second driving unit

What is claimed is:
 1. A ferroelectric recording medium including anelectrode layer, a ferroelectric recording layer, and a protection layerformed in this order on a substrate, wherein the ferroelectric recordinglayer includes a ferroelectric layer and a paraelectric layer, theparaelectric layer being disposed between the ferroelectric layer andthe electrode layer, the paraelectric layer includes one or moreparaelectrics selected from the group comprising oxide, nitride,carbide, boride, and silicide, a lattice constant of a materialconstituting the ferroelectric layer and a lattice constant of amaterial constituting the electrode layer or the substrate arelattice-matched within a range of ±10%, and a lattice constant of aparaelectric constituting the paraelectric layer and a lattice constantof a ferroelectric constituting the ferroelectric layer arelattice-matched within a range of ±10%.
 2. The ferroelectric recordingmedium according to claim 1, wherein the ferroelectric layer is asingle-crystal film.
 3. The ferroelectric recording medium according toclaim 1, wherein the ferroelectric layer has an amorphous structure withshort-range order, a distance of the short-range order is equal to orless than 2 nm, and a lattice constant of the amorphous structure andthe lattice constant of the material constituting the substrate arelattice-matched within a range of ±10%.
 4. The ferroelectric recordingmedium according to claim 1, wherein the substrate includes silicon, andthe ferroelectric layer includes hafnium oxide.
 5. The ferroelectricrecording medium according to claim 4, wherein the ferroelectric layerincludes a mixture including hafnium oxide and one or more additivesselected from the group comprising silicon, aluminum, gadolinium,yttrium, lanthanum, and strontium, or a mixed crystal Hf_(x)Zr_(1-x)O₂,where x is 0.3 to 0.6, including hafnium oxide and zirconium dioxide. 6.The ferroelectric recording medium according to claim 5, wherein acontent of the one or more additives is 1 atom % to 20 atom %.
 7. Theferroelectric recording medium according to claim 1, wherein a filmthickness of the paraelectric layer is 1 nm to 100 nm.
 8. Theferroelectric recording medium according to claim 1, wherein a filmthickness of the paraelectric layer is 1 nm to 30 nm, a film thicknessof the ferroelectric layer is 1 nm to 30 nm, the film thickness of theparaelectric layer is equal to or less than the film thickness of theferroelectric layer, and a difference between the film thickness of theparaelectric layer and the film thickness of the ferroelectric layer isequal to or less than 10 nm.
 9. A ferroelectric storage apparatuscomprising: the ferroelectric recording medium of claim 1; a conductiveprobe configured to write information to and read information from theferroelectric recording medium; a probe slider configured to cause theconductive probe to travel by floating above a surface of theferroelectric recording medium; a ferroelectric recording medium drivingunit configured to rotate the ferroelectric recording medium; and arecording-and-reproduction signal processing unit configured to processa write signal and a read signal of information transmitted to andreceived from the conductive probe.